It's a gas exchange thing. In theory, if the glass top seals off too much of the tank, you don't get adequate O2
exchange at the surface.
I've yet to see a glass top that totally seals things off. But one that restricts things too much, coupled with poor surface agitation, could set up a situation where your pH may slowly decrease.
My top is kinda like CaptainAhabs... the front 2/3 of my tank is glass from one of those Perfecto hinged tops, but without the hinge. The back 1/3 is totally open. (I have HOB
equipment.) I've never had pH issues.