It's only a small difference (103 degrees versus 104 degrees in water),
and I believe the usual rationalization is that since F is more
electronegative than H, the electrons in the O-F bond spend more time
away from the O (and close to the F) than the electrons in the O-H bond.
That shifts the effective center of the repulsive force between the
bonding pairs away from the O, and hence away from each other. So the
repulsion between the bonding pairs is slightly less, while the
repulsion between the lone pairs on the O is the same -- the result is
the angle between the bonds is a little less.
