I think that's pretty common with certain forends for the 870, IIRC some of them even come from the factory that way. As long as the forend isn't binding on the receiver when it slides over it, functioning should not be affected. It may partially cover the bottom loading port when the action is open, but I think you have to have the action closed (forend fully forward) when loading the mag tube anyway, so it shouldn't block the loading port when you need access to it.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)