It is not the way of the Scripture to talk about a situation until a certain point and then switch to another situation. And when it stops talking about one who has issue lying in a bed and then starts talking about one who has issue sitting⁴⁰⁰, there is a homiletical interpretation.

Leviticus 15:6 says,והישב על הכלי אשר ישב עליו הזב יכבס בגדיו ורחץ במים וטמא עד הערב(And he that sits on any thing whereon he that has the issue sat shall wash his clothes, and bathe himself in water, and be unclean until the evening), changing the situation fromמשכב*(lying in a bed) in the previous verses to*מושב (sitting), and thus breaking Rule 173⁴⁰¹.

footnotes: ⁴⁰⁰ Leviticus 15:6. ⁴⁰¹ Malbim, HaTorah v’HaMitzvah Metzora 134.