credits and time

hi, I use chaos cloud several times now and today I render in the cloud 4 times the same scene (skp.) with the same settings…

what I was noticing:

render 1 : 1st time 3,8 credits in apx. 38 min. (4K), 2nd time (still running at the moment) 1hour 40 min. and running… 9,7 credits… (also 4K)

render 2 : 1st time 2,9 credits in apx. 29 min. (4K), 2 nd time : stopped after 1hour 16 min and 7,6 credits… (4K)

I like an explanation for this matter, this really make no sense and I feel betrayed…

regards
cedric