Top horizontal is the 一 radical. In 不 it is the sky-line that the negating wings push down against; this top stroke is also why the char is indexed under 一 in the Kangxi system rather than under any of the lower strokes.
ideograph
不bù
no; not (lower-body remainder)
The three lower strokes traditionally depict a calyx of a falling flower or a bird flying upward refusing to land; that downward-pointing shape became the abstract negator. Treated here as a fused remainder because no standalone CJK char captures just the lower portion.
