Alright, so I chased down the difference. For uninteresting reasons, the box with the "geod" discrepancy is ancient, and was running proj 4.8.0. By 4.9.0 the discrepancy was gone. The commit that fixed it was this: https://github.com/OSGeo/PROJ/commit/f1f1f9f2aea725663499f449961027df2b38e296 Thank you very much, all.