Most application windows (firefox, pcmanfm, libre office, etc) allow you to resize them in pixels.
Most (all?) terminal emulators only allow you to resize them in characters - e.g. 80x24.
On my XFCE system (with a 2560x1440 screen), I have a top panel (menus, short-cut icons, some status displays) and a bottom panel (taskbar, desktop switcher, status bar, etc). Between those two, I can fit a terminal window that is 192 characters wide by 51 characters high. There is a small area (maybe 4 or 5 pixels) that is not covered by the terminal window.
The combination of my font setting "Monospace Regular" @ 16pt and 192x51 characters requires 2513 pixels by 1336 pixels (so says
xwininfo). The handful of extra pixels is not enough to have an extra line (if there was, I would use it).