Bias in Rio swimming pool

Researchers told the Wall Street Journal that results seem to indicate swimmers in one half of the pool had a performance boost in the 50m, which has competitors swim one pool length.

.
https://www.theguardian.com/sport/2016/aug/18/olympic-pool-current-swimming-results-rio-2016

I've noticed the same effect in every pool in which I've swum. One half is always faster than the other half. It appears to me that the top half of the pool has a distinct advantage over the bottom half.