NTP Forged Packets?!

My internal ASA and ESX host are pointing to NIST for NTP. The packets I'm receiving at 4 minutes faster than the actual time. I've tried several different NTP servers, and still 4 minutes off. Laptops offsite have the correct time, and it screws up domain authentication. on the ASA it thinks it's getting good time info, and all the VMs are in sync with that no problem. So is Comcast forging the NTP replies to me?