Changing map values eith the percentage function

Hi everyone, Im new to tuning, I'm using a haltech sprint RE, for a 13b turbo(series 5). When using the percentage function to change values in the base fuel map, the maths doesnt add up. Like if the value in the cell is currently 10, I press P and enter 50% , the value doesnt change by 50%. It seems to change by different amounts different times. Can someone explain to me how this precentage function works? I have a seperate table for injector deadtime btw

How far off from 50% addition is it? I would expect 10 + 50% to give you 15. What are you getting?

If you're getting a value like 14.8 or 15.2 or similar, perhaps the table resolution isn't high enough to enter the exact value?