Bill reflects Gigabit 1000 speeds like I upgraded to, but noticed Fast 500 is listed under my services tab but still only receive <250 mbps

Hello! Thank you for taking the time to read this inquiry. I’m not sure if this is an account specialist, technical, or billing issue/reques. Nonetheless, here are the details:

About 4 months ago, I upgraded from the 25mbps prepaid internet plan to the regular postpaid monthly Gigabit 1000mbps tier. 
I have the xFi modem that was given to me from an Xfinity retailer store when I upgraded to Gigabit speed about 4 months ago (XB7 Technicolor 2.0 with firmware CGM4331COM_6.7p14s1_PROD_sey).

I have noticed a slight increase in speeds but not anywhere near what I had anticipated so after several countless hours of research, I can confirm that I am receiving <250mbps download speeds (per Ookla Speedtest measurements.. Xfinity internet speed test says I’m getting 1136 mbps download speeds but I know that is incorrect). Also, tonight I happened to notice that under my “Services” tab in the Xfinity app, it has “Fast” 500mbps speeds listed as my internet tier even tho I having being paying for Gigabit 1000mbps internet speeds for the past couple of months (as I had requested and my bill reflects). I have changed my internet tier 3 or 4 times over the last 7 years of my consumership with Comcast, but I’ve never signed up for this tier of internet speed, ever. I don’t know where this number has come from but it causes me more reason for concern and speculation as to what is going on with my slow download speeds given that I’m paying well over $130/mo. Can someone help me figure out why I’m being billed for Gigabit 1000mbps internet speeds (as I had upgraded to and am paying for) but have Fast 500mbps tier listed under my “Services” tab but yet still only receive <250 mbps speeds and how to go about getting the 1000mbps (or close to it, I understand and have read the fine print) that I have been paying for. Thank you so much for your time and help.