Extra latency at ATT exchange for UVerse

Can anyone explain why ATT's UVerse adds significant delay to packets compared to their ADSL service? For example, pinging 8.8.8.8 from an ADSL gateway shows a latency of ~10ms. From an UVerse gateway, it's about 40ms. Of the extra 30ms, about 10ms can be explained by the fact that UVerse last hop is interleaved. ADSL seems to have Fastpath enabled more often than not (at least in my city). The extra 20ms is more interesting. By pinging each hop obtained by tracerouting to 8.8.8.8, the extra latency seems to be added on the exchange between ATT and Google. It's not just for 8.8.8.8. The same holds for other hosts too. ATT seems to add 20ms when it hands off a (UVerse) packet at an exchange. Thanks, Srikanth