Answer: Old windows were made of glass spun out into flatness. That's the center of the spin.

New windows are made of glass floated on a pool of molten tin, which makes very flat panes without any annoying bullseyes.

EDIT: this is also the source of the myth that glass is a liquid and flows over time. See the center pane of your picture? Someone took the time to get a "good" pane for the center of the window, without any bullseye - put your best pane in that spot, obviously. But the glass is still of uneven thickness, getting thicker closer to the bullseye. So people look at panes like that, with thicker glass in one part, and decide that "hey, the glass must have started off perfectly flat and flowed over time". But no. The glass flowed when it was molten (1500 degrees C!) and does not flow at room temperature.