Xfinity Arris Wireless Router/eMTA Speed Question

I believe I have the Arris TG862 eMTA/Wireless-N router. I have the Extreme 105 Mbps package. I have found that when I connect wirelessly to the router, it will only produce speeds of 50-75 Mbps on a good day. Usually, it is 30-50 Mbps. However, if I hook my Linksys E3200 wireless router to an available ethernet port of the ARRIS router, any device connected wirelessly to the Linksys E3200 will achieve 105 Mbps speed. So my question is this: Is there a reason that devices can't get the same speed on the Arris? Another poster also told me he couldn't achieve advertised speeds via wireless but did when he was hardwired to the Arris. I think it is pointless to have two wireless routers going at the same time. Plus, I believe the radio signals are interfering with each other because the Linksys signal regularly drops out.