I think it's more likely that the low speed resulted in the power being lower. After the initial analysis when the connection is established, the tones that are determined to be unusable, will have nothing or only a very low nominal power level transmitted on those tones. If you have lots of unusable tones, then the aggregated power level totalled over all the tones, might be lower than that of a line that is able to make use of more tones.
The maximum permitted power level for a particular tone would be determined by the band plan. If, when transmitting at that maximum power level, the SNR on that tone isn't enough for it to have any bits allocated (because line is too long and the attenuation of the signal at that frequency is too high, and/or there's too much noise), then there isn't really much point in continuing to transmit on that tone.