F₂ C=C=CF₂  bcs. continuous double bond planes are perpendicular to each other,so F containing c being sp² have their F in  perpendicular planes.

In case of allenes the terminal carbons are sp2 hybridised and central carbon is sp hybridised. Thus the atoms of one carbon are in the plane of paper, while the atoms of other catbon are perpendicular to the plane of paper.Thus in the first compount it will not be possible for all the four F to be in one plane.