I tracked the change to the following commit:
```
commit d34e7f282dbe676ffc93e6aa79e7ff64966cfc40
Author: Rachel Mandelbaum <rmandelb@andrew.cmu.edu>
Date:   Sun Feb 10 21:32:56 2013 -0500

    fixed tests to work with new keywords
    (#238)
```
Here is the discussion near where it was added, which doesn't give much insight:
https://github.com/GalSim-developers/GalSim/pull/351#issuecomment-13367551

Ah, but the previous commit had it back at Quintic again...  Tracking back further, the original change seems to be at:
```
commit 1506e9c02f3927d857b90862029c72532b685b90
Author: Rachel Mandelbaum <rmandelb@andrew.cmu.edu>
Date:   Fri Jan 4 11:13:19 2013 -0500

    interpolant stuff
    (#333)
```
The discussion about is is here:
https://github.com/GalSim-developers/GalSim/issues/333#issuecomment-11893004

It seems that it was changed in order to have the default conserve flux, which Lanczos doesn't (quite) do.  However, I suspect we care more about fidelity of shearing accuracy than flux conservation, so I think we probably want to go back to having Lanczos5 as the default and just point out the issues with the flux.

Also, I think Gary just derived the flux correction from a Taylor expansion type of calculation.  We might be able to figure out what the next term is to improve the flux conservation properties of Lanczos...