@pagapov
I assume that You want fluid / resizable layout that will arrange Your widgets vertically even, with the horizontal item indent.

I don't know why Your Ui bahave that way. I tested with stretch and difference is only 2px. Here is my test Ui: aboutdlg.ui
and PNGs (with "test markers")
http://www.imagebam.com/image/8a479a103526056
http://www.imagebam.com/image/6e962d103523909
http://www.imagebam.com/image/aa3425103523910

As for the clue, I don't have any , the only reason AFAIK for this to happens is that QVBoxLayout arrange items by their height and QHBoxLayout don't return correct height for the items inside it (from brief look at the src it actually return sizeHint so probably I'm wrong on this one). So it treats QHBoxLayout as expendable horizontally and thus the bigger size for this layout. But as I said, it's only untested theory, please correct my if I'm wrong, or simply see implementation of the Q[H/V]BoxLayout.
I stumble upon similar behaviour with splitters and nested layouts so stretch fixed behaviour for me partially, there was still around 5-10px "free space" for particular splitter with layout.