I have a similar issue with an ebay-purchased Computrainer. It's reading ~100W higher than my Powertap.
Where on the CT unit is the adjustable pot?
I'm going to run my other bikes over the CT to confirm that it is the CT that's incorrect before I adjust anything but it's well out of warranty so I'm not too worried.
EDIT:
"OK, I'll reply now that I can again....
I'll reiterate -- on our dyno, these units read correctly at the calibration point. Lab units are verified over a broader range -- but none of this will account for slippage or calibration routine errors, which almost always occur. We are displaying power from a load curve that operates the Load Generator and is based upon speed. The same errors people have indicated in speed variations from other devices to CompuTrainer can be equated to this too. The Load Generator is producing the correct load for the speed it sees and displaying the correct wattage for that speed. Period. If there is slippage, we have no way to factor that back into the equation except to use a constant [or assumed curve] of what that would be, which we know is neither. That said I'm going to let everyone in on a little secret -- you can adjust the CompuTrainer if you are so inclined. This adjustment will only raise and lower the curve, but not change the curve shape. DISCLAIMER: If you make this adjustment and then subsequently need me to put it on the dyno to recalibrate it -- it'll cost you $75. Proceed at your own risk. To adjust the CompuTrainer remove the sticker on the inside edge of the Load Generator [on the side closest to the tire]. This is the one indicating the power supply to use, etc. Behind this sticker is a series of rectangular holes. In the uppermost left hole is a potentiometer. Turning this pot to the left lowers the load at a given speed. Turning this pot to the right increases the load. If the CompuTrainer reads low on the watts reading, then chances are the load at the Load Generator is too high, which is limiting your speed, which results in a lower wattage display. Roger
RacerMate Inc."
http://www.postfinder.com/post/24419438.html
and more discussion here:
http://www.cyclingforums.com/t/319578/srm-vs-computrainer-wattage-discrepancy/15