I think that is an effect of the WrongAngleFixer class which tries to reduce
the distortian effect of rounding to Garmin map units.
Polygons are not treated in the sam way as lines.
This problem is also on the todo list:
Have noticed an interesting 'feature':
I add outlines to buildings if they have a name.
in lines : building=yes & name =* [0x1100A resolution 24 continue ]
99% of the buildings have the desired outlines , but every now and then something unusual happens with outlines showing the odd triangular peak, ie not matching the building polygon.
Somehow the line & polygon coordinates do not match?
You can see the effect here : www.pinns.co.uk/building.jpg