Remember that MPLS tagged networks report the ping time as the RTT for the virtual circuit. ICMP is also low priority, and, on a busy router can be dropped or delayed. Also, some routers enforce rate limiting and when ICMP gets over a certain threshhold, it'll drop/delay those packets as well.
Additionally, you could be experiencing an asymmetric route. Traffic that goes to that one router may return through a different route which may have more hops or congestion somewhere other than the path you see. So, when you hit that one AT&T router, their internal policies, or Comcast's route announcements may prefer a different ingress than the router the packet just exited.