We used to have an aircraft (not a 737) that did this, and it was "always like that" with nobody curious enough to determine why, as it was easily controllable. Then a newhire, doing his first preflight on an actual aircraft, discovered that the 50-70 odd vortex generators present on the right inboard flap did not appear to exist on the left flap. This D-model aircraft had been flying around with a C-model flap for years. Some strong-willed mechanic got it to fit, and that was that.