H-bridge output voltage
Yes, this kind of voltage drop is normal for this part. Look at the values for VOH and VOL on page 4 of the datasheet. The high-side switch has a typical drop of 1.1V @ 0.5A and the low-side switch has a typical drop of 1.0V.

As for why your two motors are different, perhaps one of them is binding a little bit in one direction, and drawing a bit more current that the other.