OK - they are rounding very badly.
The 20' radius should look like
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
Rather than
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
OOOOOOOO
i.e.
52 squares instead of 44 squares
Yielding a Pi of 3.25.
Which is a lot better than the 2.75 we got previously.