FM10-Align Top Edges of Vertical Lines

[Mandu]

I have a set of vertical lines arranged horizontally (think: fenceposts). I want to align their tops with the same imaginary horizontal line. I select them all, and click Align Top Edges, thinking this will ... well, align the tops of these lines. Instead, I am seeing them move downward a few pixels... and when I click ALign Top Edges again, they again move downward! As if I were repeatedly hitting the down-arrow key.

It's like Birnam Wood creeping toward Dunsinane.

Is this actually behaving as it should??