Document estimate when resharding
What changes in Document estimate when resharding compared with MongoDB and what your application has to do.
This difference is of the Different kind.
What MongoDB does
approxDocumentsToCopy comes from the fast count of the donor shards.
What TeselaDB does
It is exact for collections under 1,000 documents and a size-based estimate for larger ones.
What your application has to do
Use it as an estimate of progress, not as an exact count.