Best guess is that it was serviced but the indicator was not reset? Not sure what the service intervals are for the petrol engine, the diesel is 12500 miles or 6 months whichever occurs sooner, the petrol may be the same? All of Mazda's service records are held digitally (no books to "stamp") so a dealer would be able to confirm, assuming that it was done by a Mazda dealer?
Had my 18 month done yesterday, but I'm not doing my "normal" mileage these days, so only at 32,000 currently . That being said, I've never had the reminder come up, the dealer always contacts me before its due.
Alan