It has happened to me several times that when rolling a 7 my turn is only 15 seconds long, I believe I have tracked this occasional behaviour to happening when I started that turn by robbing with a knight.
Other factors that may influence whether it happens:
Opponents having to discard half their cards?
But when I rob with the 7 my "1 minute turn" timer never starts, it's just counting down from the 15 seconds. I can extend this by 15 seconds with a trade but I would not get the 1 minute initial timer.