re. stroller

Question

I am wondering what the difference between this case of p'sik raishe and combing one's hair on Shabbos. With regard to combing we look at each stroke seperately, therefore it is not a p'sik raishe, like the Rabbi holds in his Tshuvos. I am wondering therefore why a stroller is any different, if you look at every push seperately it shouldn't be a psik raishe. Please clarify or make a chiluk. Thank you, and I love all the decent posts!