Hi Guglielmo,
Steve is on the right, er, track.
It's normal that the two channels don't balance when listening to 2-track tapes with a quarter track machine.
You might think that quarter track heads would line up with half the width of the recorded tracks of 2-track tapes, but they don't.
The reason is that there are unused guard bands between the recorded tracks. In the case of 2-track tapes, the guard band is almost as wide as each recorded track, so the tape is essentially divided into thirds. The outer quarter-track lines up pretty well with one of the 2-track tracks because it is near the edge of the tape, but the inner quarter-track (track 3) is off quite a bit. This is shown in the attached sketch.
If you run the numbers for actual track widths and positions, it turns out that about 87% of one quarter track head gap covers recorded material on a 2-track tape, but only 41% of the other gap covers recorded material. With less signal, there is less volume. So it's no surprise that one channel is louder than the other on playback.