Volume eval looks wrinkly

This looks wrinklier than I recall:

Magnitude-wise, the same phenomenon may also explain the (relative) inaccuracy we're seeing at the moment. This is with a target-stick-out-factor of 0.005 (but the wrinkliness doesn't look like it only occurs on the last 0.5% of the center-covered region).

The figure can be reproduced using examples/layerpot.py at 0967b85c.

cc @mattwala