Chris,
The specification on both batteries is a peak current (for a short period) over 500 amps for a new fully charged battery. I highly recommend you stop using the CCA tester. It produces significant currents internally for the small batteries. Even at 10 second duration, it may weaken inter-cell wiring or cause damage to the plate and glass mat internal to a cell. If you really need to use the big load, and you can get 500+ amps or better when you first pull the trigger, I think the battery may be OK. Just don't leave it on for 10 seconds.
We have been using the CBAIII for many years and to match manufacturer specs. We set for 7 amps and 8 volts terminal voltage which is the MK graphs cutoff. I tried to test batteries last week only to find that the CBAIII is not supported for Vista but West Mountain will take it back and refurb to correct the problem. If you own one, see the West Mountain site for details and RMA info.
I should add that high resistance connections may also be giving you false readings. Improper crimps or loose hardware on the battery terminals could show up as low CCA numbers. We use a #10 external tooth star washer between the battery and the wire terminal. This will cut through any surface crud on either terminal and prevent the two terminals from sliding around and loosening the hardware.
West Mountain does sell a higher load device if you need to check larger batteries but the extra load is six times more than a CBA.
http://www.westmountainradio.com/cba.php