I do not have the unit you discuss. But here are the questions I would have to answer:

1. Is the discharge circuit actually an accurate 'constant current' discharge during the entire discharge session?

2. If the answer to number 1 is 'yes' then what is the exact discharge current rate during the session?

3. What do they use as a timer?

4. What is the cutoff voltage for the discharger?

You say the readings are off by 15%. What are you using as the 'standard' for comparison? In other words, off 15% compared to what?