I by no means am an expert, but wouldn't you want your list properties to be set as a class, rather than a id?
ul.list-menu {
in html : <ul class="list-menu"...
rather than
ul#list-menu {
As for your question... perhaps your list is picking up a margin from another style? I think the experts would ask to see the complete stylesheet, and html to determine that.... maybe?
I myself have been having trouble with lists and bullet images and found a way around it 
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