[USR-TC] Line Quality Question

An interesting issue I've discovered. Here's the situation: I have a computer with a hardware-based US Robotics 33.6K modem. If I dial up to my rack on an analog phone line connected an analog nic of a Quad V.34 Dig/Analog modem, I get 33.6K EVERY time. In fact, EVERY SINGLE time I get 33.6K, no matter how many times I dial. With the same computer, same modem, same phone line connected to the computer, if I dial up to one of my Hiper DSP's, I can't get better than 28.8, and usually 24K or 26400. Does this mean that signal quality on the T1's into my Hiper DSPs isn't as good as a regular POTS line? How should I interpret this? Is this anything I should be concerned about? Just curious. Thanks, Joel