Skipped / missing invoice numbers

I'm using Reckon One.  Every now and then it seems like there is a tiny lag in the system while I'm invoicing. It appears to try to catch up but nothing more than a second or two. When invoicing sometimes it skips an invoice number or today, 2 consecutive numbers in a row. (#942 & 943 gone). I can't find the numbers anywhere and I haven't created any adjustments/credits etc. Straight invoicing to a variety of customers. It doesn't seem to be a problem, as saved invoices are never missing. But where are the missing invoice numbers and why does it happen? It looks like I've deleted invoices but haven't, and it makes my consecutive invoicing look weird seeing that there are some numbers missing.