More Lane Bias

I did a lot of thinking about the directional lane bias that I accidentally discovered after poking around in my Garmin after my Tuesday the 26th swim. The pool swims fast going west and slow going east. At least if does in the north lane (I think they call that lane two). First, I postulated that the pool was unlevel and going west you swim downhill and turning around is an uphill swim. Then I figured maybe not. 

My next guess was the speed difference had to do with the earth's rotational  spin. As the earth spins east, it meets us as we travel west. Whatta ya think? 

Then I did another swim test. I swam in lane one and this time I also discovered directional lane bias, but it was the opposite. While lane two is fast going west, lane one is fast going east. That kills the unlevel pool and the earth's rotation as an answer. 

With my top suspects eliminated, what could the answer be? I sat in at the end of the pool and thought as hard as I could. I thought so hard my head hurt. I thought so hard I became hungry. I thought so hard I grew angry and wanted to smite someone about the head and shoulders. While I fumed, the pool water gradually calmed and then I saw it. I saw it! What did I see? Answer in the comments. The person with the correct answer will receive a yet-to-be-determined prize.