It could be a number of things that is slowing your connection. While Comcast may offer a "Maximum" of that speed, your area may not support those speeds yet. However, your router, modem and ethernet cables play a HUGE role. First off, make sure you have a DOCSIS 3.0 modem. The most common modem, the "Motorola Surfboard" is a DOCSIS 2.0 modem, and only supports download speeds of around 50Mbps. Next, make sure that your router offers Gigabit ethernet ports, and fast wireless speeds (at least advertises 300Mbps, though you wont actually get this). Next, I would update your ethernet cables to CAT6 cables. That is your cheapest option here, but your biggest difference will come from updating your modem.
If you rent your modem or router from Comcast, they usually put in their new combo modem/wireless router box which is DOCSIS 3.0 compatible. If you rent from Comcast and do not have this, bring your equipment into a local office and get updated stuff. They'll give it to you for free.
Honestly, your speeds are fine, but faster speeds never hurts!