Garmin speed/cadence sensor

Yes, it's possible. 1 mph isn't even all that odd for them to be off, either.

Speed on the trainer can be highly variable due to tire pressure and resistance.

So true - one of the reasons I'm VERY glad I have a HRM now.

As an example - my workout sunday? Average HR of 156, average speed 18 mph. My workout yesterday? Average HR of 165, average speed of 15 mph. It totally depends how much pressure I put on the tire when setting up the trainer.

The computers probably have different tire sizes.  You can calibrate the Garmin outside using GPS.  Non-gps computers estimate based on your tire size (you can do this with the Garmin too).  As other people mentioned, the only way to compare trainer rides is with the EXACT same set up, sometimes pretty hard.  If you want to try to compare, look up in your old computer what the tire size was set as and use the same in the Garmin.  I try to keep things similar, but with my Power meter and my last two 20 mintute tests, I had 10 more watts but the exact same speed.  No change in equipment, just differences in tire pressure and tightness of the trainer to the wheel probably.

Was your test inside or outside?

Either way, in both cases 1MPH compared to a different day is really hard to track.  Inside, tire pressure, resistance unit position, and gearing will all contribute to different speeds.  On a trainer, you can achieve any speed/toughness combination you'd like simply by modifying variables.

Outside, wind (even a bit), road conditions and tire pressure will contribute as well.

In theory, if everything is identical, then both using the same circumference would be identical.  In reality...that's really really really tough to do.