technicall question about d3 cable modems speed issues

hello i cant understand some problems in my isp networks
lets say i have legall d3 cable modem cisco epc3010 with 120mb speed
in reality i got just 30mb in night and in day 15 - 20mb
when i take that modem and going to my friend far away from me i got on his cable line speed up to 60mb
this cable modem in my house bond just to 1 channel download and 1 upload 1x1
with d1 cable network in my house everything is fine no glitches tested with uncapped modems running even up to 50mb cause i have eurodocsis and speed is allways same
my question is why this is happens ? is there are some isp limitations with d3 network in some areas ?
even if yes then for sure d1 network is 80% much overloaded with old clients than d3 which is almost new
the signal levels i have as follows
downstream channel 1 -8.5 dbmv , signal to noise ratio 37.6dB
upstream channel 1 power level 57.0 dBmV
the rest of channels are totally off
i did read in some websites and people around confirming they had similiar problems they pay for 120mb and got only 60mb some of them got like they should 120mb , calling and stumbling isp doesnt change basically nothing they send the techinician into yours house he check something and smile and goes away
thanks for any hints