Petruza wrote:
But it does horizontal scrolling ( obviously )...
...as well as vertical! Since the game has two types of scrolling, and only one type of mirroring can be used at a time, one type of scrolling is gonna take the hit. Some programmers prefer to introduce vertical glitches, others horizontal. In SMB3, notice how the left 8 pixels of the screen are masked (to hide glitches) and the left actually shows color glitches as the screen scrolls. Like you said, the screen on the right is a mirror of the one on the left, so as soon as tiles scroll out from the left, they are introduced on the right. The glitches exist because the attributes must be changed in front of the player's eyes. The 8 pixel mask can hide tile glitches though, as tiles are 8 pixels wide.
In fact, as tepples said, the mirroring doesn't dictate the type of scrolling used. Mirroring is just something the programmer has to take into consideration when designing the scrolling engine, but any type of mirroring can be used for any type of scrolling, even if there are a few glitches visible as a result.
With vertical mirroring, a horizontal (only horizontal!) scroller can be programmed without any visible glitches. But if for any reason the programmer wants to use the second name table for something else, say, a status screen or something else he doesn't want to be overwritten by the scrolling, he might very well use horizontal mirroring and use only 1 name table for the actual game, which will introduce glitches at the left and right of the screen (unless tricks are done to hide them).
So, "horizontal scrolling" is not interchangeable with "vertical mirroring", even though this is true in many cases. I do think that it would make much more sense if we used Nintendo's terminology though, since many people get confused because the H and V on the boards do not match our mirroring terminology. We'd have to stop using the word "mirroring" and start calling it "arrangement", like tepples did.