Good connection to xfinity, terrible connection beyond that.

I have 600Mb service, and when I run a network speed test connecting to a comcast server, I consistently get 600+ Mbps.  But, in every day web surfing, my connection is slow.  When I run a network speed test pointing to any server but a comcast server, I average usually around 3 to 4Mbps.  Is this normal for xfinity?   Why pay for 600Mb service (or even 200Mb service) if i can't get anywhere near that in the real world?