Have you tried measuring the voltage right at the terminals of the power supply, and then at the cells, using the same meter? Any difference is the resistance drop in the leads. If the external reading at the power supply terminals does not agree with power supply internal display, it either means the calibration of the internal meter and your meter do not match, there is a substantial resistance between where the power supply is measuring the voltage and the output terminals, OR it could be a combination of the two.
I would also suggest testing the power supply at a low current and see how accurate the output voltage is. What is your desired tail current? Load the supply to that current and adjust for the desired voltage. Then just leave it on the cells until it reaches your tail current and the voltage is then your desired absorb voltage as well. Yes, it may take days with several large cells in parallel.
I used a 600 watt power supply to balance up my NCM cells and that still took me a few days. NCM and LFP do act a bit different, but the goal is similar. You want to get all the cells to full charge. NCM cells don't fall back, they stay at the absorb voltage until you apply a load. LFP cells will slowly fall back a few tenths of a volt when the charge source is removed. I have not personally worked with large LFP cells yet, so I can't say how much this impacts the process, but from following Will and a few others on here, it just looks like you need a lot of patience. When the cells are all connected in parallel, they will share the power and self balance to some degree even without a power source connected. Even in the flattest part of the discharge curve, the cells will pull each other to the same voltage, and if the cells are at the same voltage and temperature, they must actually be at a very similar state of charge. As you pull the voltage into the knee area, the difference in state of charge will certainly close up and bring the balance together even more. When I did a parallel balance, I actually used a 10 inch piece of #14 wire to each cell so that I could measure the drop on that wire to see what was going on. I had a buss bar on all of the negatives, but the light wire on the positives. When I first connected, one of the cells was high and I saw a fair drop on it's wire, and about 1/3 of the drop on the other three as it was giving all of them some charge. These were only 4 amp hour A123 cells, and it was pushing over 3 amps out of the high cell. They all reached the same voltage in under an hour. Obviously, 280 AH cells will take longer. A LOT LONGER. If the currents are high, you can use a clamp meter on the wires, but measuring the millivolts across the lengths of wire works just fine. I used those cells for a 2S2P RC car pack. Went right back to LiPo as they A123 were not even close in performance.