Every possible train goes into HPs. CF's leveling doesn't care if your con is maxed naturally or through gear. Like int and wis it uses the highest value. Most sets of gear have enough bonuses to cover a few missing points. I'll sometimes train dex or str if it is way, way below the racial max, like on an arial or dexy human at roll, but usually not more than two trains. Add up all the HPs you can get by pushing as many trains and pracs into HPs as you can, and it starts to add up. By how you're describing it, you are losing at least 50 HPs, which on a mage is about 10% of your base HPs. Your char gets 13 trains by level 51, that ends up being 130 HPs on my chars and I think the majority of players do the same thing.