Permalink setting: should read gallery, but i get gallery-2

I’m sure this question has been asked before, but I searched through the forum comments/questions and did not see anything.

When i change the permalink from the default to a pretty one, i sometimes get a hypen and then a number. Most of my links look great, but one named url/gallery comes up url/gallery-2.

Why is this happening? How do i get rid of the hyphen extension