I know this sounds crazy but do you breathe differently? Some people alternate which side they breathe off of per length and you could just be faster breathing to the right rather than the left. Also, just like everyone else has mentioned there can be a noticeable current in a lot of pools. When they turn on the jets at 6:15am you can tell. I can even see it in training peaks.

There is a definite clockwise current in my regular pool. It is more significant in the outer lanes (those lanes get extra turbulence from the side walls/gutters too). It is typically 3-5 seconds difference per 50m length in the outer lanes (1 & 6) and slightly less in lanes 2 & 5. I rarely swim in 3 & 4 but I don't think it is as noticeable in 3 and 4. One of the lane 3/4 swimmers noticed it when he had a rare swim in lane 1 for a 5000m swim where he was maintaining a steady pace. He doesn't use a watch so any small difference in direction in lanes 3/4 aren't really noticeable when looking at the clock but it was noticeable when he was in lane 1.

Do some longer sets with a watch and you should be able to see a general pattern after a few swims. I ignore the first 50m of each set since its always faster for me and then look at the results. At my pool its a pretty clear pattern.

The water is thicker

---

"What's your claim?" - Ben Gravy
"Your best work is the work you're excited about" - Rick Rubin