Hey,

As Fluorine is more electronegative than chlorine, therefore it can attract a shared pair of electron more easily and strongly than chlorine.

The anamolous behaviour of fluorine's electron affinity on the other hand is considered to be so because of the small size of Fluorine molecule.

As fluorine is such a small atom, putting the new electron into a region of space already crowded with electrons and there is a significant amount of repulsion. This repulsion lessens the attraction the incoming electron feels and so lessens the electron affinity.

Thank you and keep asking.