According to a site with extremely questionable methodologies and software numbers... Microsoft, themselves, said that Forza 5 sold 1 million on February 18, 2014, including digital sales. Why should I believe that it sold over a million retail copies on the week of Jan 18, 2014 when it has been proven otherwise?

Well if you read the gamespot article about that announcement it states that the game sold over a million copies by the begining of Februray already either way the numbers aren't off by much and its safe to assume it sold over 2 million physical copies already. 

Sorry... what 1 million in Feb 2014 have to do with "assume it sold over 2 million physical copies already".

Stop to talk about something you don't know.

VGC is overtracking it at the 1 million milestone and there is zero proof about 2 million sold lol

 Again read the article that came with the announcement it says that it sold over a million by February already so the numbers aren't off by much either way and that was over a year and a half ago already it selling another million since then isn't a reach.  Also there is zero proof that it hasn't sold over 2 million already so lets just leave it there since this discussion isn't going anwhere anyway.

You're stretching the data a bit too much, don't you think? The gap between January 18 to February 1 is nearly two weeks. Assuming Forza 5 sold 20K per week, VGC would have overtracked by about 42K sales. In addition, Microsoft's number includes digital sales, so actual retail sales are 1 million minus whatever many digital copies that were sold. It is extremely difficult to pinpoint the percentage of people who go digital-only, but MCV cited a study that found only 10% of UK gamers go digital-only. If we assume 10% of Forza 5's sales is digital, then 900K retail copies were sold as of the beginning of February. That would be approximately a 150K overtrack. If it's 15% digital, 200K overtrack. And this is why VGC's software numbers are very questionable.

There is also zero proof that is has sold over 2 million. We lack evidence that stronglys suggests for or against. I have not heard Microsoft making such an announcement nor has it made a statement like, "The Forza series has sold over XX million". If we get at least the latter, then we could estimate Forza 5's sales. Just because there is no proof for either direction doesn't mean you assume the positive. While MS released a Forza 5 bundle, it got overshadowed by Titanfall in March and the Kinectless SKU in June. It did not sell well in any of the other months of 2014 until September. However, the uptick in sales was attributed to Destiny.

The article says over one million by the beging of February it doesn't give any specific numbers.  Also again there is zero proof that it hasn't sold over two million copies already.  Shortly before Horzion 2 came out before the game got its free weekend the leaderboards where over 2 million already.  So including digital copies it was close to two million almost a year ago already.

The "there is no proof for X, so therefore the opposite of X is true" argument is not a valid one. Stop using the Appeal to Ignorance fallacy. You also need to prove that each individual in the leaderboard has his/her own Forza 5 copy, whether it be physical or digital. The information available to us is not conclusive enough, though VGC might have overtracked Forza 5's physical sales based on my reasoning in my previous comment.

I'm not saying Forza 5 didn't sell over 2 million, but I'm not saying it did either. If I were to take a wild guess, it has sold just over 2 million including digital sales.

I'm pretty sure it sold over 2 million physical copies already since its pretty much been below its launch price for over a year already.  With digial its pretty much over 2.5 million already since its been on sale digital numerous times already also.  That beig said its sales will completely dry up in less then six weeks since once Forza 6 comes out there is no reason to buy it anyomore since 6 will pretty much have all the content of 5 and a ton more.

Digital sales being 20% of the total is too steep. I heard digital console game sales make up betweeen 10 to 20% of the total, though I cannot find the article at this time. Also, VGC likely overtracked Forza 5's sales based on what I said earlier, probably by 200K. Assuming physical sales are 1.8 million and digital sales make 15% of the total, then total sales will be 2.11 million. If you really want me to be generous and assume that VGC did not overtrack, then total sales will be 2.35 million.

It was downloaded over one million times when it had its free weekend I'm pretty sure a good chunck of those people bought the game instead of just deleting it after the weekend was over.