prob

The central atom changes.
As we go down the group the central atom becomes less electronegative.
So, in the top of a grp, the electron pair will be more towards the central atom which decreases (i.e. becomes further than the central atom) as we gradually move down a grp (because the electronegativity decreases).

Now more closer the electron pair to the central atom, more closer they will be to each other and hence they will tend to repel, so the bond angle will be more

Hence the bond angle decreases down a group