[Zonx]

Objects are positioned left to right and top to bottom in the order selected. If, for instance, you select the following objects

A B C D

in revers alpha order and hit distribute horizontal, it will revers their order during the alignment, resulting in:

D C B A

Selection order can be altered by re-clicking an already selected item. For example if you select the following:

A B C D B

and hit distribute horizontal, you get

A C D B
By re-clicking "B", you moved it forward in the selection order, telling distribute to place it last.

Same deal for vertical distribution, with earliest object selected moved to the top, last selected moved to bottom.

Edit: UIBuilder's ditribute is based off neighboring edges. Objects of differing sizes will have the space between them equalized. The space between their center points will vary depending on their sizes.

There doesn't appear to be any way to get centerpoint distribution.