Simulacrum Splinter Count Seems Random?

What actually determines the number of simulacrum splinters that drop? When I full clear the map and shatter all the mirrors, sometimes I get 40, sometimes I get 8. It really seems random.

Also, every time I clear the map and after killing the boss, the delirium bar on the bottom of the screen has a turquoise fractured mirror on it located at the very far right of the bar, implying I never shattered that fractured mirror, but its never anywhere near the boss, or nearby. Am I missing something? I have never shattered all the fractured mirrors in a delirium because of this. I have the entire delirium atlas tree allocated.