difficulty is difficult to actually set, as a certain way of playing and certain methods can make parts of the game very easy on any difficulty level.
the main thing that changes when you change difficulty is economical, on rookie, enemies develop slower, there are fewer of them in every mission so you have more time to set up your teams and its easier to successfully complete missions for their rewards (that include more SP to develop troops with). you also get more starting resources and the troops you recruit come with gear, this means you get up to speed quicker, as you don’t have to build everything for the troops you can instead use that production power for other things. it also allows you to shortcut to certain techs by reverse engineering “bought” weapons and equipment.
but the enemy…while lower in number…isn’t actually any weaker.
this means 2 major things with regards to difficulty, difficulty in PP seems to revolve much more about how quick you can progress (can you level and deploy fast enough to stay ahead of the pandoran development) and second any trounce combo you might find in a lower difficulty can still totally wreck a higher difficulty (as an enemy dead on rookie with a certain weapon+ skill combo, is equally dead on legend)…it will just be harder to get to them as you’ll have to build/buy or train more.
the third major player is that the game has a number of very powerful combo builds, and geoscape strategies that can rocket power their economy, this means that if a player on a lower difficulty finds one of these combo’s and breezes through the game, he can do it…with an economics speedbumb at the start…almost the exact same way on a higher difficulty. for players that find these combo’s the game might seem very easy, for players that don’t find them or don’t use them the game can be notably harder. it creates a disconcert in experience.