1080p minus 30%(ish) = 900p, yeah. (900p: 1,440,000px - 1080p: 2073600px). It's usually easier to work upwards though, so 900p + 44% = 1080p.

He might be thinking of 720p. 720p + 125% = 1080p.

The nearest you get to 1080p, the better the (upscaled) result will be. It's better to go to highest you can, even if it's an unusual resolution than to stick to a straight 720p that is only half the resolution.

And for why not directly a 1080p, as far as I read, the slow memory of the XOne requires to make a good use of the very fast esram, which is also very limited in term of space. The better the resolution, the more you will need memory or slow down the process with transfer from/to the main memory. That's a balance they can't really overcome, because what ever gain they get from optimization, they are using it to draw better graphics or improve framerate (for a better result, but also to be on par with the PS4).

Gran Turismo 4 for PS2 could output to 1080i (not 1080p but close enough when it comes to PS2). My point is, almost every console could theoretically output games at 1080p. It depends on the games. If every Xbox One game were much simpler in terms of graphics then every game on it would be 1080p. 

As others have explained 792p and 900p are less pixels pushed by the GPU. As PS4 GPU is ~41% stronger than XB1 GPU, we often see this gap: 900p / 1080p or 792p / 900p which is explained by the power difference between 2 consoles.

Now why 912p (Metro redux) instead of 900p?

Why 792p (Watch_dogs, Titanfall) instand of 800p?

It's because some resolutions upscale better than others. 792p is better upscaled to 1080p than 800p and 912p is better upscaled to 1080p than 900p.

792p and 1080p are divisible by 72, hence the 792/1080p ratio is a perfect 11 / 15 ratio.

912 and 1080 are divisible by 24, hence the 912 / 1080 ratio is a perfect 38  / 45 ratio.

The lower the perfect ratio, or the higher the greatest common divisor (gcd) between native to upscaled resolution, the better the upscaling will be.

1080i = 540p (in a manner of speaking). You need to know what progressive and interlaced means.

1080i interlaced means that at any given frame only half the image is sent to the 1080p display. the next half comes in at the next frame. And by half they mean Fist half = all the odd numbered lines that make up the image and second half = all the even number lines.

I mean it's not hard to do the math.  But yeah it was extremily rough lol my bad.  1080p is a 44% larger resolution.  768p is really what is half.

The mystery is solved! Thats been bugging me for a while

I would say 1080i is a lot better than 540p in almost every case, I do know the differences between interlaced and progressive. But I take back my statement nevertheless, I just read several places on the net that GT4's resolution is just 640 x 448 with a line doubler, made for old HDTV's with bad upscalers.

And you would be right, hence why I said "in a manner of speaking". They both ahve their drawbacks. 1080i while giving you an image with more pixels (technically 1080) only gives you half of that image per frame. So you end up with jaggies by default. 540p (though in truth they only ever used 480p) gives you an image that lacks the jaggies but isn't as clear overall. But its all good, you do seem to know about all this anyways so no need to go into more details.

1080p vs 1080i and others have nothing to do with internal resolution rendering.

They are related to the way the screen shows every frame. All those process are a little complicate to explain.

Native resolution has became this generation battle ground for the console wars. But wasn´t a thing that most developers were conserned, because in most cases it doesn´t affect game play.

Unfortunally, almost one year after there are still endless discusions on foruns about images that people couldn´t tell the resolution they are, even side by side.