The problem is that while your connection has improved in terms of bandwidth to your ISP, the network between your ISP and the UT server hasn't changed.
One thing that most new broadband users don't understand is that pure ping numbers aren't likely to go down to quite as low as they think they will, but that your major performance gains will be in the overall reliability of the connection. That is, you'll get a much steadier low-end ping.
Now, as for the difference between F1 and F6 ping: The ping displayed with F1 is your actual honest-to-goodness ping. This is the time in seconds that it takes for a ping packet to travel to the server and back.
However, pure ping doesn't mean much in a regular game. There are other factors, such as the time it takes to process that packet, both by you and the host server, the slowdown due to rendering a frame, etc.
The F6 number you get is what epic calls "effective ping" and takes into account things like the server's current processing load, your computer's current processing load and graphics draw time, a little bit of averaging to get you a more "realistic" number.
For those of you in America, where this gets mentioned, think of the F1 ping as the current temperatue, and F6 as "windchill" which is how it feels to a person outside. F1 is true ping, F6 is "adjusted" to what it "feels like", and therefore will almost always be higher.