Your imax charger seems at fault. Testing these kind of guys is tricky. I think the most common method is to batch-test in a way I'll vaguely describe:
If you have the right tools you can use a battery analyzer, or a recharge capable charger, and speed up some of this. The discharge process is really slow on most lipo chargers since it has to get rid of the heat within its case. So you do what you can to minimize that part of the system.
One of the best methods I think for fast discharging and testing the cells is to remember that batteries in series discharge the same current. So charge up your batteries to full charge, then make a rig that has a connector for a battery monitor and a few halogen lightbulbs (bulbs are great for radiating energy more easily than a convection/conduction heatsink - but just a resistor and heatsink works too!). Now discharge your cells in series until one reaches a cutoff voltage, and stop. You now have equally discharged the batteries so if you want to go the rest of the way you can use the charger to measure and add whatever it takes to recharge the worst battery (which is roughly at 97% efficiency). Fancier rigs will include a wattmeter in the setup. You might also get by with simple qualitative testing, weeding out the cells that seem to die quickly compared to the rest in the series connection, or that have high internal resistance (I recommend getting an IR meter if you can afford), and assume the remaining are suitable.
I dont know thats a quick rightup, but the truth is Dr.Angel has got a million pages documenting recycling 18650 cells. Bon voyage. I have done it once but am fairly confident i will not go through the effort again (shit ton of work..but also appealing on a recycling and cheapness perspective.)
To reiterate, you're right. no way in hell those batteries are all discharging the same amount.
