SPDIF has no clock signal independent of the audio samples. If you have a device that outputs SPDIF digital audio going into a SPDIF input and set the receiving device as a slave in the ASIO control panel, the receiving device's internal clock will be ignored and the clock is interpreted from what is being received on the SPDIF input. I have an 18i8 and this was the only way I could use the SPDIF output from the Behringer Bass V-Amp Pro in my setup. I never checked if I had weird sample rate stuff like what you are describing but I did have issues with a pop in audio now and then that I could not trace down by looking at the audio waveforms like you can usually do. I assumed it was due to slaving the 18i8 to the SPDIF. Since selling the V-Amp I have not had the issue.