Because the way sprites are loaded in Factorio means the "most restrictive" sprite definition wins and a given sprite is put into that atlas.
This specific decorative is used by a 'decorative' and a 'decal'. Decals go into one atlas type and decoratives in another.
However, because this sprite was used by both - the decorative using this sprite had to render from a different atlas compared to all other decoratives.
This broke batching almost completely on vulcanus resulting in an excess of 6000 draw calls each frame.