Windows don't resize to touch the bottom of screen

This has bugged me for years, and I was wondering if anyone else had this problem, or if Apple intended it. I like to have Safari and a few other applications like Mail take up the whole screen from the menubar at the top down to the bottom of the screen (with dock hiding on), and from the left to the right side. However, my windows will never resize to the last pixel on the screen, there is always what seems to be 4 rows of pixels where I can see my wallpaper between the bottom of the window and the end of the screen. I know those 4 rows of pixels don't make much difference, but it just bugs me having that strip of wallpaper at the bottom. Is there a fix for this, or is it permanent?