When some results in a search result set do not have a value set for the sort index, those results are excluded by the sortResults() method in Catalog.py. This is a longstanding behavior, and isn't entirely unreasonable. However, the result count attached to the final results (actual_result_count) is not adjusted for the excluded results, so the length of the sequence may be shorter than its reported "actual" length. See:
https://github.com/zopefoundation/Products.ZCatalog/blob/2.13/src/Products/ZCatalog/Catalog.py#L769
The simplest solution would be to decrement the actual_result_count variable in that except block, but I'm not sure if doing so would be entirely safe or cover all cases where this issue might arise. There are a number of code paths based on different optimization strategies here.
Fixed via decrementing actual_result_count in https:/