James is correct- as with every transformer I have installed, yes, the taps will drop when loaded, exponetially so at a nearly 95% load. It would actually be very desirable for the 12v tap to drop down to 11.5 or so for your close in lights to the trans, just for bulb life. but the 15 volt tap should read anywhere from 14.5 to 15.5 at around 120volts in my experience, loaded or unloaded. I would send the unit back and tell them the 15v tap is wired identical to the 14.
as an update, after 4 years and 3 months all the ryco units I installed continue to work as I have described in all previous posts. I have not installed a new one in quite some time, as I have went to hadco/phillips when looking for a smaller value trans, as they cost about the same as ryco and have a closed door system, and carry a lifetime warranty from a known company. you do have to spend extra on a photocell/timer with the Hadco/phillips, but then they are easily replaceable while the ryco's are not. The photo-cell is the one item I have had issues with the ryco's as stated in previous posts.