Gif files are a “dumb” format and just do whatever they’re instructed to do.
The internal rotation of it does not talk, look or sync with anything.
I you want more control over the individual slides you should re-do the advert as a HTML5 advert. That way you can influence the slide speed for example. But that’s probably more hassle than it’s worth.
It wouldn’t surprise me if the gif’s slides are simply more seconds (5.5 or 6s). Or the timing in your browser is off compared to the server.