Checking super capacitor ratings

[Wiggys.shop]

So I got a set of 6 new Maxwell capacitors and wanted to check the capacitance of them. I used the following procedure.
https://youtu.be/DbPiQKTJntU

I came up with 400 F instead of 500 F. I did the same test at a higher current of 5.1 amps and between 13 to 15 volts and got 497 F. I am guessing im not checking them correctly but I'm not sure what my mistake is.

Thank you for the help.