User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:126.96.36.199) Gecko/20070324 (Debian-188.8.131.52-4) Galeon/2.0.2 (Debian package 2.0.2-4)
If "Hide frame on windows when maximized" option from Window Manager Tweaks is unchecked maximize will wrong and won't cover whole the avaible area.
To reproduce it, is also needed to remove menubar, toolbar, borders. (i checked with xterm)
Steps to Reproduce:
No, xterm sets a size increment to maintain a window size multiple of the size of a single character, it just happen that your screen size is not a multiple of the size of the caracter size.
This doesn't show when the frame is hidden because in that case xfwm4 will ignore the size increment and set the window to the real available screensize, but that is a violation of the size increments. This violation is made because it would look ugly if the window was not totally enlarged w/out the window frame.
In other words, what you describe is not a bug, it's the standard expected behavior.