I'm fortunate enough to have plenty of solar that getting every last Ah is really critical for my application, although "getting what you paid for" would certainly be nice. I don't know why the big variance. Is it the tester, or the batteries??
Probably both and also BMS balance leads should be in the equation.
Unless you want to loose time comparing voltage from the ZKE, JK BMS with an multimeter and has long you are happy with your system and you don't have a very low cell on discharge or the same cell high while charging don't bother with it, since you are in this forum probably in a couple of years you will start to replace your cells with a new tech or more performance one