Other differences I could see from the screenshots: the comboboxes have the dropdown arrow next to instead of inside the edit control, the table headers should have a thick left and top border too, the up/down control's buttons should also have a 3D border, the scrollbar buttons shouldn't become disabled even at the end of travel, and non-top-mounted tabs have their shadow in the wrong direction and side tabs should have text rotated accordingly. I've used this UI for over two decades so it's pretty easy to see when something doesn't look quite right.
Vector fonts are made of lines that can sometimes be smaller than a pixel wide. Font engines solves this in different ways. It both depend on the quality of the font and the settings of the engine. (I'm no font expert though so names might be off. But I have been playing on and off with pixel fonts and vector graphics since 30 years)
This is one reason I use Xfce. My brother once told me that my computer has looked basically the same since I used Gnome 2, and he's right.
I'm not entirely immune. After switching fully to GTK 3 it took me a while to find a decent style that had scroll bars or not terrible scrollbars. The one I'm using right now (ClassicLooks) has a few issues (e.g., I can't tab through GUIs because it won't highlight the active option), but overall is acceptable. I'll fix the highlighting issue eventually...