Missing Orders?

Since last weekend we have a series of missing orders. If you look at the attached screen shot you can see the numbers are no longer sequential and I have no explanation for this:

[CENTER]

[LEFT]Until then sales orders always appeared in strict number order so if you wanted to count the number of orders in a period you simply took the difference between the first and last order with all these missing transactions it is impossible.

I have checked to see if this is a checkout abandonment but the customer would have to first register and then we would have corresponding leads created. It could be a PayPal issue because it is PayPal that creates the account but as we have been using PayPal for years I cannot see what has changed last weekend.