[GAMEPLAY] "Strength of the Ages" mastery not giving 20 health on large monster kills

In one of my recent games, I played Zac jungle and took the "Strength of the Ages" keystone mastery. However, I noticed that the mastery was giving me ten health per large monster kill rather than the 20 that is says it gives. It still stacked up to 300 health total and then capped, but it took twice as many large monster kills to get there than it should. The mastery is supposed to give ten health on siege minions and 20 health on large monster kills, however it was giving ten health for both large monsters and siege minions.