[mbrubeck]

"Lines of constant bearing" (or "rhumb lines") depend on the choice of poles.

A rhumb line relative to true north looks straight on a standard Mercator projection, but can look like a spiral on another Mercator-style projection where the pole and center-point have been swapped.

[mkehrt]

Oh, that's an interesting point. Maybe that's what's going on. It's hard to picture such a line with a different pole.