Before we dig into what's happening and why, we should put this filtering problem into context by showing how it affects other textures that are less likely to show obvious seams between mip maps. Here's a pretty good example of how much this problem affects most textures in the game.
You can see some minor aliasing in one of the texture seams at the top of our sample images, but that's about it. There are more mathematical differences between the images that show up in the full "diff" image, but they're not massive. Whatever's going on here doesn't harm image quality too greatly, but it does cause problems in certain cases.