If they guarantee something, they are bound to fix it if it's out. So they set themselves somewhat low standards. Of course the movement can perform much, much better (and most of them out of the factory do so). But if it runs at -9 or +19 seconds, they can still claim to be within their standards.
Furthermore, this raises the question of what exactly these values relate to. Most people would measure accuracy by the amount that the watch gains or loses per day. However, as I always state, fixing this is a matter of pushing a lever or turning a screw. If my Zenith had such large
deviations around the mean daily rate (i.e. if it was running at an average of ±0 seconds but some days, it gained up to 20 seconds, others it lost up to 20 seconds), I'd be pretty disgusted! But noone ever asks about that - they are happy if the watch is running at an average of ±0 seconds.....
Hartmut Richter