Skipped numbering in account move

I installed Tryton 7.4 to start using it again. I configured the basics — a client and a product — and created two sales invoices. When I checked the accounting moves, I noticed that the first invoice does not follow a sequential order; there are some gaps, whereas the second invoice has its moves in order: 2, 4, 5, 6.

Is this behavior correct?

The numbering of account move is not strict so it may happen especially if some lock mechanism is needed by the process creating the move.
Anyway this number has been replaced by the posting number in 7.6 series.