TWC doesn't even use DOCSIS 3.0 capable hardware yet, but that is exactly how their caps work. I'm sure Comcast does the same thing.
Their Roadrunner Turbo service gives you (for example) a 10Mb/s connection, but files below 100MB in size will transfer at 15Mb/s. Speedtest will
show you have a 15Mbit/s connection, but really it's capped at 10Mbit/s.
TWC recently increased their line rates but haven't updated their info to indicate what exactly the cap is now at, I think it's 15/25 Mbit download, 2Mbit/s upload but I'm not entirely sure because it keeps chaging. I just did another test and the results are even more strange than the last.
I tried downloading that file, it started at 2,200Kbit/s then dropped to almost exactly 1,800 for the rest of the transfer. At least until the power surged and everything but the computer died.
Total transfer time was six minutes though next time around...
Roadrunner also says the same thing about the rates taking time to "set in" with the modem, I presume it has more to do with them getting their entire infrastructure updated and up to the new speeds, as well as the modem needing to power cycle once for the changes to take effect.