“Pending sectors” refer to sectors that failed to be read. They are marked that way so the drive knows they could be unstable or even bad. And they are kept that way as long as there’s data stored in them.
When at a later point the sector(s) become unused, at the next write operation, the drive either removes their “pending” status (if the write is successful), or re-allocates them (also removing their “pending” status but increasing the “reallocated sectors” counter).
So if you’ve seen that value increasing from 2 to 40 and then going back to 0, you should now check to see if any other smart counters have changed (especially the “reallocated …” ones). If those are still are 0, there’s no need to worry about the drive, it could all have been due to some less-than perfect cable connections. If this happens again, consider changing sata/power cables.
