Maybe one was rated at 2.83v and the other at 1 watt. If they present different loads to the amplifier, that might explain it. Or one speaker might be rated in free space, the other in half-space or quarter-space. Half-space represents about 3dB over free-space, and quarter-space is about 6dB. Some companies rate in free space, and others measure on a large baffle, which is half-space. Another thing that could be is that one might just be a little bit conservative and the other a bit optimistic. Then again, I suppose something could be just plain ol' out of spec.