Digital Recorders

I have a Roland VS2480 connected to a PC via a co-axial cable to transfer audio digitally to the DAW. When I set the digital clock source to the PC's co-axial input everything is hunky dory. If I disconnect the PC and use the internal digital clock in the VS, it plays back at a slower speed. Why?

Tascam's two new digital multitrackers are the latest in their long line of Portastudio products.