Do you similarly consider going 56 in a 55 zone the same as doing melachah one minute after shkiah on Friday evening?
driving 56 in a 55 IS illegal, however they dont usually give you a ticket for it because they allow for the radar gun to be off a bit or your speedometer to be slightly off.
You could certainly say the same thing for a clock and perhaps your clock is a minute or 2 off and Shikia is not exact (Like you get the Shkia time for NYC (Manhattan) and you live in Brooklyn, there could be a minute or so difference