Thanks Dan.  It just sort of konked me on the head - duh, the moon 's full orbit takes 29.something days, so it will reach perigee and apogee within that time period (easily two weeks).  With the earth orbiting the sun and carrying the moon with  it, it  tugs the moon a little to and fro as well.  For this annular the earth has "kind of" thrown the moon a little farther away, so we get an annular eclipse rather than full one.  Does that make sense?