Like tball said, there should be two weights attached together. When you set the reference tension that you want, you are supposed to line up the measurement on the "yard stick" with the base of the weights.
Given the above, are you saying that you set the reference tension to 55 lbs and your measuring device said it was 59 lbs? If that's the case, I don't have an answer for you.
The arm doesn't have to be level but it shouldn't be extremely angled either. Something like a 30 degree range would be ok.