I have read it was a remnant from the 737-200. As stated above, the heat exchanger in hydraulic system A had less capacity than the one in hydraulic system B, and hydraulic system A's heat exchangers are coupled to the left main tank (tank 1). But then.. what is the reason?
The underlying reason for this (and now this is my theory), is that it's because of the hydraulic hiding in system A. The landing gear is coupled to the hydraulic system A and you see the hydraulic quantity indicators showing less for system A when airborne than on ground because part of the hydraulic fluid is trapped in the landing gear system.
With less heated hydraulic fluid in system A there is less capacity to heat the fuel.