I go back to my previous post and re-iterate, it is down to the sensors not compensating for whatever the density is, unlike on the 75/767 that I have also flown, which do. On them, when the tank says full (valve shuts whilst fuelling) the guage reads full quantity, not short or over, but full.
It matters not what the actual density is because 737 fuel guages will only read correctly at whatever the 'design' density was. Knowing the actual density of the fuel you are loading will still not get your guages to read correctly!
PP