Rollover not filling whole block.

I have a weird issue. I have added CSS code to nice_menus_default.css to change the background color on rollover,

  ul.nice-menu li a:hover {
    color: #fff;
    background: #000;
  }

which does indeed change the background color, but not for the whole cell. For some reason the top few pixels stay the current color.

I have attached an image so you can see what i mean.

Can anyone point me in the direction of where i am going wrong?

Cheers

Gareth