Crossbows don't generate in pillager outpost chests, even though they have a 100% chance to spawn in outpost chests

If I've read the loot table correctly, the chance for a crossbow to appear in an outpost chest is 50%.

The game selects a random stack out of a pool. In this case, the pool only consists out of a crossbow – so it will always select the crossbow.

Each pool can be chosen from multiple times. How often is also randomly determined. In this case, the game "rolls" the pool at least 0 and at most 1 times. Hence, it's possible that it doesn't select an item from the pool at all. So, no crossbow in that chest.