Why does the Torah write "asher Yishkav alav " and "asher Yeishev alav ... " (in the future)?
Rashi and Targum Yonasan: To teach us that it the article is only subject to Tum'as Mishkav if it is designated exclusively for lying on or for sitting on, but not if one needs to ask the person who is lying or sitting on it to get up and allow the owner to use it for whatever it is designated for ('Amod ve'Na'aseh Melachteinu!').
What if the Zav does actually touch the article on which he is lying?
Rashbam: A Zav, a Zavah and a Nidah who are lying or sitting on a stone 1 which is placed on top of a bed or sheet, which he is not touching, adopt Tum'as Mishkav or Tum'as Moshav, to be Metamei a person and the clothes he is wearing.
And the same will apply to the bottom sheet, if he is lying on top of ten sheets.