Well, see when you scroll down, so that said moving advert is no longer visible - is it still moving? Like, up above the screen, is that advert still happening?
If you scroll back up after 10 seconds, and the advert is 10 seconds further on, is that because it knows it's visible again, so it goes "I'm going to start playing again 10 seconds further in", or has it been playing along without you?
Does the bit of the computer that puts the effort into making the advert go still put that effort in even though you've scrolled down?

I realise I've asked the same question a few different ways here, but wasn't sure how to describe it. I've decided to just ask it as many ways as I can.