[_ck_]

the deleted post will always causing missing numbers

that is because in your template the number is based on post-position which is never updated

the important part is that the count changes from 21 to 20

the question is why when you run the direct recount within bbpress itself, it did not change that 81 to 80, but the manual query changes 21 to 20

if you want those numbers to be continous on the page, you’d have to change your template but it doesn’t affect how bbpress works