Getting a little more information now.
I ran a search on her phone and it returned back, say 6 emails - all of them I'll call Zombie emails. Clicking on any of these emails resulted in being told the message couldn't be found on the server.
Now, let's say that the latest of these emails was from August 6th and was 'found' in the Inbox. At that point I look at AquaMail and it tells me there are say 310 unread emails out of 355 (she has a thing about marking emails as read - don't ask!).
Now, what I did find is that if I went in the Inbox AND started to go back in time in the message list to the point where older emails were being brought back THEN that 355 count started to drop down in value as it refreshed the internal count based upon what it was really seeing. In addition I was unable to see the emails that the Search reported. If I went back to the search THEN it would report non found (because the internal list had been updated possibly).
What I think might be happening is that there may be emails that she delete on either the Gmail web site or on her phone (using AquaMail) which were not 'seen' as being deleted on her tablet when it next reloads emails.
Any of that make any sense?

I've not tried running a Compact operation yet - I'll try that next.
On the plus side, I finally convinced her to buy a Pro license!
