Kylie, it might be you just have a different scale of difficulty compared to most people? I've never played mortal kombat this gen, so I can't say how hard it is. But perhaps you just have a different scale from other people... For example, what games would you say warrrants a 9/10 or 10/10 difficulty?
Though of course it's entirely valid people are just voting out of their asses... Reading over the trophy list, some people are stressing over win 10 online ranked matches in a row, when PSN's been down (though I can't imagine that'll be easy regardless)
edit - as for the time thing, I'm sure roadmap writers on ps3trophies.org take that into account, so the time they post will be accurate. As for people factoring time into their difficulty votes, I'm not sure how to account for that. Hopefully someone has an idea.
edit 2 - I just checked myself, the guy who wrote the trophy guide specifically mentions the leave on overnight trick. He says:
This is the total objectives that you need to complete OVERALL MINIMUM:
- 2,800 wins
- 2,800 -alities (fatalities and/or babalities)
- 4,200 X-Rays moves
- 280,000 liters of blood
- 672 NET HOURS!! of play (like 28 days playing a full 24hrs a day)
There are two tips for making this trophy half way faster and doable in around 350 hours or less
So the leave-on-overnight method is projected to cut the time in half, and that's already been taken into account. On top of this, it's still estimated at 350 hours.