For smooth structured data on a polar grid produced from a simple analytic function, isocontours are wiggly. When using GNUPLOT to produce isocontours from the same data, isocontours are smooth. Therefore, I suspect a problem with the isocontour finder.
The GNUPLOT result can be reproduced by typing:
gnuplot> set contour
gnuplot> set cntrparam levels 20
gnuplot> splot "asy.dat" using 1:2:3 w l 0
For the same data file, 'asy < test.asy' produces the equivalent figure. Setting '--' to '..' does not make a difference.