>1) I'm assuming that Numero is some kind of identity/sequentially assigned value, and that the rows you're trying to delete will ALWAYS have a numero value that is less than the numero that's being assigned on the insert

It is the primary key.

Yes, records to be deleted are under that value.

>2) Are there insert/update/delete triggers on the Hit table that you're inserting/deleting?

No trigger is applicable.

Numero is also an index and part of one combined index. I am not sure if this could be a factor here.