: Don't "invest" in comic books. With that out of the way...
It's supply and demand. If you're looking at the same exact issue (as your question implies), some sellers may be out of touch and listing at a price higher than the market demands, and others may be looking to sell quick by undercutting the market average.
There are also examples of issues being over-graded, which is why you buy the book, not the label. If you're looking at different issues in the same grade, well, there are multitudes of factors at play there. First appearances, cover art, etc.