Why am I being charged for international data usage when I paid a flat rate of $200 ($10/day) for the Global Travel Pass?

My wife and I recently traveled abroad. We did not want to get saddled with data charges while in Europe and were meeting friends who lived there so we needed to use our phones. We purchased the Global Travel Pass for both of our phones and used them on the trip as one would expect (maps, looking up restaurants, texting). We were charged the $10/day, but also charged for data usage by the MB, which seems totally wrong. My wife was told that we went over the 512 MB cap, which is why we were charged extra. But no such 512 MB cap was explained to us over the phone. The website also contradicts this charge--which says that once you go over 512 MB, "data is available at 2G speeds until a new pass is started." Nothing says you will be charged more if you go over 512 MB. The pass is advertised as a "flat rate for calls, texts, and data." I don't see the point of paying $10/day (for something that is free with other carriers, by the way) when you will simply be charged by Xfinity anyway by the MB.