The windscreen and roof don't seal together. It is designed to let a significant amount of air through (between the top of a windshield and the roof), presumably for circulation during the warmer months.
I filled the gap for the winter by using several insulating tubes that you'd use to insulate your copper plumbing -- the stuff that looks like a pool noodle. In fact, some people have actually used pool noodles, not only at the roof/windshield line, but other places on the machine (e.g., around the roll cage in spots).
You're not going to get a car-like seal with the Pioneer cab. If that's what you are hoping to do, let me be the first to let you down. Sure, you can do your best to stop much of the wind and elements, but you're never going to get it completely sealed. There are endless threads on sealing up a Pioneer on here.