73 40 99 67 22 45 78 71 45 99 5 46 18
Assume these are the next RNG rolls you will do in your game, but the game obviously doesn't tell you. Their values are set in stone even before you take the first shot.
Whenever you need a random number, XCOM will take the first item off the list, and remove it from the list. When a new random number is needed, it gets the next item. And so on.
Note: if you take a 50% shot, any number over 50 registers as a miss. So a 45 would be a hit, and a 52 would be a miss.
The below is an example of what would happen to the same turn if you were to continually reload and try again.
Scenario 1:
Sniper takes an 70% shot. (Roll = 73, therefore it misses)
Grenadier takes a 35% shot (Roll = 40, therefore it misses)
Ranger takes a 100% shot (Roll = 99, therefore it hits)
RAGEQUIT! RELOAD! AGAIN!
Scenario 2:
Grenadier takes a 35% shot (Roll = 73, therefore it misses)
Ranger takes a 100% shot (Roll = 40, therefore it hits)
Sniper takes an 70% shot (Roll = 99, it hits)
Note how the two scenarios played out differently, even though the same units took the same shots, and the RNG rolls were exactly the same (as is the case for XCOM). Even though the rolls were decided, your order of operation was up to you all along. You just didn't know what the RNG rolls were going to be.
If you were to know the RNG rolls before they happened, you could always shoot with the unit who will hit, but without using overkill (e.g. not wasting a 100% shotgun blast on a shot that has an RNG roll of 2, because anyone with >2% hit chance will hit that shot anyway).
This should also show you that if you RNG roll a 99, even if you knew it was a 99 beforehand, it would be impossible to hit your enemy unless you had a 100% hit chance. The same applies to a low roll; a RNG roll of 10 will be converted into a hit by any unit who has over 10% aim.
Footnote:
For the sake of clarity, I assumed only shots take a number off the list of RNG rolls. It would be more correct to say that a number gets used anytime a random number is needed (to see if a shot hits, to determine weapon shot damage amount, to determine grenade damage amount, ...). Therefore, by taking the actions that need RNG in a different order, you can change the outcomes.