Is this a Thai wife or Retirement extension of stay (or some other) and was your entry on 16th using a non immigrant O visa from a Consulate or using a re-entry permit from immigration?

Thai wife and re-entry permit from immigration.

Now that explains it - you appear to have made the application for extension prior to travel so new reporting date would have been 90 days from your TM.6 entry. It is the application for extension that counts as start of 90 day count - not the actual issues of final extension of stay.

Ah ok, yes, indeed. Makes sense after all. Thanks