I wondered the same thing, but if you look at the angle as an acute, not an obtuse angle, then it becomes clear.
Having two lone pairs pushes the atoms away in H2O, which explains the 105 degree angle.
One lone pair pushes atoms away less than two lone pairs, which is why the it's 120 degrees for a standard sp2 orbital.
Finally, a single electron pushes atoms away far less than two lone pairs would, which is why the bond angle is greater than 120. I believe the explanation says something close to 135?
Hope that helps!
