Help me with maximum golems !!! Is this a bug ?

Hi, can you guys help me explain why I can’t summon 16 golems when they’re mixed?

In the video, I was able to summon the maximum of 16 Fire Golems, but when I mix different golem types together, I can only summon 15.

Video: https://www.youtube.com/watch?v=CMZ1nFJZ_G0&feature=youtu.be

this is my pob: https://pobb.in/lSdIaCPj5tr_