When two, or more, hoppers are connected horizontally and fed two or more items via a chest or minecart the first hopper has one item in its inventory until all items have passed through (like it should). But, when the hoppers are connected vertically, the first (top) hopper "pulses" a single item in and out of its inventory. This occurs in both survival and creative modes.

A curious side note: The vertical setup's "pulse" does not create a redstone signal whilst the single item in the horizontal setup does produce a redstone signal. This is evident in the attachment, where both chests have one stack of stone. This screen shot was taken approximately five seconds after inserting the stone into the chests.