How is the modem provisioning boot file related to upload/download speeds?

I'm having problems with upload speed on a new Arris S33 modem (this is the one recommended by Comcast).

The initial provisioning didn't work at all, and after a tech fixed that problem I'm getting 700+ Mbps download but only 5 Mbps upload. I've verified these speeds with both speedtest.net AND Comcast's own speed test website. They give identical results.

I presume the allowed speeds are part of the configuration sent to the modem (the boot file?) as part of provisioning.  Is that the case?  What else could cause the modem to upload at 5Mbps when download is fast?  The modem's signal strength page doesn't show any anomalies... upload channels are all around +46dBmV.