Why is a shared counter sometimes too small after several threads update it?

A read-modify-write sequence is a logical transaction, not automatically an atomic application operation. Interleaving can overwrite another worker’s update. Protect the complete update with a threading.Lock, or collect per-worker counts and combine them after joining. Test deliberately synchronized competing updates rather than depending on a rare timing accident. Do not use assumptions about a particular interpreter’s global lock as your data-integrity contract; extension code and runtime configurations vary. A lock protecting one assignment is insufficient if an invariant spans several fields. Keep critical sections short and establish one lock order when multiple resources are involved.