Outer beam positioning on stave line

1. Open attached score (produced in 1.3).

Discussion: In page 18 of 'Behind Bars', it talks about the outer beam positioning of up-stem and down-stem notes on stave lines.

For up-stem, they either 'hang' or 'centre'.
For down-stem, they either 'sit' or 'centre'.

In MuseScore, the up-stem notes 'hang' off the stave line. However, the down-stem notes 'centre' on it.

I just wanted to ask Werner: What factors resulted in the current implementation?

Using MuseScore 2.0 Nightly Build (5ba3947) - Mac 10.7.5.