The 56k specs were asymmetrical, the downstream connection could be 56k (actually 53k) while the upstream connection was only 33.6k. The key with v.90 was the ISP could send a fully digital signal down to the customer but they could only send an analog signal up which was capped by physics to 33.6k.

The later v.92 spec supported a digital upstream and could hit 48k upload.

Regardless, dialup users did not have a lot of upstream bandwidth available. They also suffered through high packet loss and latency making their throughput even lower than their line speed would suggest.