B757 Pilot reported a MEL relating to the fuel system was not complied with during fueling procedures and a fuel discrepancy was discovered at the destination airport. An appropriate logbook entry was made and the aircraft was turned over to maintenance.

Narrative

MEL 28-XX-X-X and MEL 28-XX-X-Y for Right Main Tank.While flying from ZZZ-ZZZ1; suspect improper fueling for the flight. Known fuel at engine start was L 11;400 lbs; R 11;500 lbs; C 0.0 lbs; Total 22;900 lbs. After landing in ZZZ1 2.5 hours later CALCULATED fuel to be (known starting fuel - known burn from FMC) L 4;0000 lbs.; R 3;900 lbs. C 0.0 lbs.. Total 7;800 lbs. (INDICATED fuel from gauges L 5;300 lbs.; R 2;200 lbs.; C 0.0 lbs.. Total 7;500 lbs.. As you can see there was a large discrepancy between the known (calculated fuel) vs. what both gauges were reading including the operative Left Tank Gauge.I asked the Mechanic to measure the actual fuel remaining in the right (deferred gauge tank) which he did by transferring all remaining fuel from the right tank to the left tank. 2;100 lbs. transferred out of the right tank to the left and then the Fuel Pump LO PRESS Lights came on; indicating the tank was now empty. Apparently the right tank only had 2;100 lbs. remaining of usable fuel compared to the 3;900 lbs. I calculated (11.5 - 7.6 used per FMC)My question is how did this happen and what can we do to prevent it from happening if this was an issue? On a somewhat related note the Left tank gauge seemed unreliable as well. During the flight it went blank for about 1 minute and then came back on. At block in the L gauge measured 5.3 compared to what I calculated should have been 4.0 (11.4 - 7.5 burn off). I wrote the aircraft up for flashing gauge on left tank as well as the discrepancy between the Totalizer and Calculated fuel.