Prestige top level goes down?

to balance them. it was a patch some time ago i think

for players with lower level than that pkmn's trainer

basically to engage more lower level players in gym fights

When prestiging a friendly gym, if a defending pokemon's level is higher than the highest level you can power a pokemon (your trainer level +1.5), then the game scales that pokemon down to that level.

As far as prestige calculation goes, this has only ever hurt me by lowering a maxed out pokemon with lower max CP to something that throws off my ability to choose half-CP prestigers. For example, a gym that looks like:
1) ~2400 Gyarados
2) ~2400 Venusaur, scaled down to ~2200
3) ~2500 Dragonite