It's because Comcast currently offers only 3 to 4 upstream channels.
It's true systemwide. Trust me.
Yes, it is system wide limit to 4. I was just researching this to see what type of modem to get for speed I use.
BTW, if you live in a Fios (or possibly other) fiber area, the upload on that is faster even for the cheapest service. Mine is 57 down and 70 up but, I have seen 90s up too. (I wish it was the opposite!)
I am in process of seeing if I can switch to Xfinity after Fios jacked up prices to pay for Verizon CEO Hamptons and Aspen homes.