Burst damage inconsistency

I notice some burst damage inconsistency when testing her out

My team is Fischl, Qiqi c3 and Beidou c6

I am using melusine weapon on her

Burst lv 8

I hit 300k+ bursts usually at 7+ stacks but randomly sometimes I hit 190-200k bursts which are crit too, idk why

Could there be any possible explanation?