Yeah it's this.
EURO emissions standards were only intended for newly manufactured bikes, but they're always used for ULEZ criteria - i.e. can you drive in a certain zone without being charged £12.50 per day...
So if you pick up a used EURO3 but not EURO4 bike, you might find (but can't predict) that in a couple or a few years not only has the zone expanded but the emissions standard has been bumped too.
Just because a bike was manufactured in a year when the standard was say EURO3, it could well comply with EURO4/5 standards, even though those standards only came about later.