why is majority 3 for one for all

I wish there wasn't a majority vote. Part of the fun of one for all is not knowing who you're going to get.

FF and OFA have two different contexts.

OFA decides what the most preferable option is among the options given. Because it's decided by preference, a simple majority works for the mode's champion select, especially considering it's supposed to be a fun game mode and not a competitive game mode.

FF is more serious because of it's game-warping nature. It's something that should allow for universal agreement on to allow a team to forfeit the game, rather than stick in a hopelessly lost game. The only reason it's 4/5 is because there are those games where one person is trolling and the rest of the team can't possibly win with him in the game, and a 5/5 vote would be impossible in these situations because that troll would just vote "no" for the surrender vote and troll the team more despite them wanting to leave the game.

It's because OFA is just picking a champ, so a simple majority (1/2) will do. 5 / 2 = 2.5, rounded up to 3. For FF, the game obviously ends and that can affect multiple things, more prominent of which is MMR, so a super majority (1/2 + 1) is needed. 5 / 2 = 2.5 + 1 = 3.5, rounded up to 4.

Because they are 2 completely unrelated things so there's no reason they need to be the same?

Its not like majority rule is some law of the universe.

imo it should be random instead

Because a surrender vote doesn't actually require a simple majority, it needs a 70% majority in order for it to pass. 3/5 is only 60%, so it won't pass a surrender vote.