I’m not a rocket scientist but I do know the two rockets use different propellants. The Atlas V uses solid propellant, and the Starship uses liquid. I know liquid engines have the ability to throttle them, and I would guess solid does not (or a more limited capacity) so that could be at play here. Starship could ignite and then throttle up, which would leave it on the pad longer. I’m sure someone more knowledgeable than I can clarify!