Greetingfs from the digital epicentre that is Wetherby UK :-)

On this page & illustrated here by pure Googlisation i got a fix to a problem which i dont understand.

Basically i wanted a solid black thick line to appear when you hover over the menu items:
arctic fox gray fox desert fox swift fox red fox

Ok so i did it with adding this bit of CSS:

#header ul a

but my question is..."Why did the display:block; declaration solve the problem"?

Any insights welcome :-)