It's possible the level cap for boosts is done in 5 train increments vs 10. This could be learned if someone has a toon of X5-X9 trainings, 55-59 for example, and tried to get +13 max health on their badge.

pretty sure its fully 10 lvl increments. I wasn't able to get +10 on a stat bonus at level 99, had to wait till level 100 before it worked.