Download speed seems locked at maximum 15 Mbps

My Xfinity statement and the Xfinity website indicate that I'm paying for 70 Mbps service:

"Internet:  PERFORMANCE INTERNET  Your Plan:  Internet 70 Mbps"

However, the Xfinity speed test consistently maxes out at a download speed of 15 Mbps.  This is over a direct-wired connection to the modem and has been consistent over several weeks, and across different days/times/devices.  It always runs right up to 15, and then stops.

My (owned) modem, an SB6141, shows a configuration file being loaded:

"SW Download INIT - Via Config file d11_m_sb6141_priceforlife12_c01.cm"

without any errors.  Is it possible that this configuration file is limiting my download speeds?  I was certainly hoping that "price for life" did not mean "no speed increases for life."  The billing statements seem to confirm that I should be able to expect higher speeds.  

I am not interested in purchasing a higher-speed plan, but would like to be able to take advantage of the full bandwidth on the current plan.