I’ll try to give you a short and simple answer.
When you run a backup, Duplicati processes your files in chunks (100KB by default). Any chunk (block) of data it hasn’t seen before (hasn’t already stored on the back end) will be packaged into dblock volumes for upload. (Duplicati never stores the exact same chunk more than once on the back end - this is how the deduplication engine works.)
Duplicati records all blocks referenced by each backup version. In this way it knows which chunks in which dblock volumes on the back end are referenced by backup versions. After some backup versions are pruned by your retention settings, it can figure out which chunks are no longer referenced by any backup version and are therefore candidates for deletion.
Because chunks are combined into larger dblock volumes (50MB by default), Duplicati waits before reclaiming space by unreferenced chunks until some threshold is reached. It may either just delete entire dblock volumes (if all chunks inside the volume are no longer referenced), or it may download 2 or more dblock volumes, repackage them (without the unreferenced chunks) into fewer volumes, upload the new volumes, and finally delete the old volumes. The goal of this compaction process is to reduce the data usage and number of files on the back end.
Technically there’s nothing wrong with deleting early. It’s just that Wasabi will charge you as if the object were still present for the full 90 days.