Staff with lyrics always has at least 7sp of distance to next staff

This issue shows up in to ways.

Context: commit 1cfa954 self-compiled on Linux Mint 14, Qt lib 5.1.0

Steps A:
1) Create or open any score with one staff with lyrics and another staff below it.
2) In "Style | General | Page" change the "min. system distance" value

Result: the distance from the staff with lyrics to the staff below changes as long as the "min. system distance" value >= 7. Any value < 7 will not reduce the distance any more. Note that if the staff has no lyrics, the distance may be reduced down to 0.
Expected result: the distance between the staff with lyrics and the staff below changes according to the "min. system distance" value.
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Steps B:

1) Create or open any score with one staff with lyrics and another staff below it.
2) Right-click the staff below and choose "Staff properties"
3) Enter a negative value in "Extra distance"

Result: the staff below the staff with lyrics will not move up above a distance of approx. 7sp from the staff with lyrics. Again note that this works correctly if the staff above has no lyrics.

In practice, it looks like a staff with lyrics has an inescapable 'wall' approx. 7sp. below it: nothing can go nearer than that.