As mentioned by many other posters, a voltage regulator in the recharging device is probably the cause for your odd discharge curve. That said, I had several comments regarding your methods that are too long to fit in a comment.
I would caution against your method of testing for two reasons:
- As mentioned in the comments, a resistor cannot change to accommodate terminal voltage dropping as the battery is discharged. This is crucial because:
- You do not appear to be controlling for a specific C-rate. A complete battery specification will advertise battery capacity at a given C-rate and lifetime in cycles for a fixed depth of discharge.
If you are "willy-nilly" discharging these batteries at 1 amp then you could absolutely going to find that some batteries are dramatically over/under rated compared to others. Discharge current causes \$I^2R\$ losses in the battery, with \$R\$ being the battery's internal resistance. I would personally perform testing at a C/10 or C/20 rate.
While I understand 1 amp would be the C/20 rate for a 20Ah battery mentioned in comments, OP has not specifically stated C rates anywhere in the question and has clearly made no effort to control the C rate.
As a last point regarding your testing - your "milliamphours" line on your plot is perfectly linear. It would appear that you've just multiplied the "1 amp" you are dissipating by the time step. As mentioned many times now, you are clearly not discharging at exactly 1 amp.
If you're not going to adjust resistance to draw current correctly, you should at least correct the capacity measurement based on the current you're actually measuring. You should measure the resistor's exact value, then use Ohm's law (\$I = V/R\$) along with the voltage you're measuring to calculate the exact current that's flowing through the resistor (and, from Kirchoff, the current coming out of the battery), and multiply that by the time step to get a better capacity measurement.