I'm a big XCOM fan so I can say for that game that the RNG is messed with on lower difficulties to protect you from a bad streak. So if you miss 3 70% shots in a row, the next one will actually give you an 80% chance, escalating as your bad luck streak continues. On non-easy difficulties the chance to hit is honest as far as I know, a 30% chance to hit is actually a 30% chance.
It is increasing your chance to hit if you've a missing streak and if you have lost soldiers. And lower the chance of the enemy hitting you if they've already hit you this turn and you have less than 5 soldiers.
So you can (and will) miss 90% shots, it should not happen twice in a row the same turn.
