This applies to every menu in the game, including the main menu. It happens in both Gold/Win95 mode and DOS mode. Here's some screenshots of the pause menu for comparison, cause that's the easiest one to see:

TD DOS:

VTD DOS mode:

C&C Gold:

VTD Gold mode:

You can see how in original TD the black background extends by one pixel past the green/grey border in all directions, but in Vanilla TD it only does so on the top and left sides.