Plastic vs. Glass bottles

I noticed something when I was trying some of my home brewed beer recently. I used some recycled glass bottles for some of the beer and for the rest I bottled it in plastic bottles that came with the kit. I recently tried some out of a plastic bottle for the first time, and the beer was noticeably more carbonated and had a much richer head to it than the beer that was in the glass bottles. I suppose this could be because I was able to get a tighter seal on the plastic bottles than the glass ones, but I wonder what happened.