If someone copied and pasted an older invoice to correct it, and your system was set to the old method where it looked back one record to determine the next number, that's why.  Then someone figured it out and overrode the next invoice # a month later.  One of the latest updates has the option to change the default "next" number to either be whatever preceeded it, OR the next highest number in that entire table.

Thank you Char. That is probably what happened.