Do a little experiment...jack the rear wheels clear of the ground, warm up the trans, place the shifter in "1", run the rpms up to 1500 and manually shift to "2". Don't be surprised if you see the rear wheels come to a momentary stop as the trans shifts to 2nd; this illustrates how much overlap there is when manually shifting 1-2. This overlap causes wear on the band, that's why it's not applied in 1st when in Drive.

Don't believe what I'm saying? Go to page 173 in Carl Munroe's 727 book and he'll say the same thing.