Here's a stab at an outilne explanation for how a higher 'working' or effective cartridge inductance might arise in the case of Grado cartridges, as per observations. For review and discussion.
to keep it non mathematical, but it's impossible really. But I'll try to summarise going along.
First step is to attempt a definition of 'effective cartridge inductance :
In these terms, 'effective cartridge inductance' as seen by the generator is simply the rate of change of coil flux wrt cartridge output current. I acknowledge that's hard to grasp in non physics terms, but it helps enormously.
Next step is to describe the Grado cartridge. Most of the description for this is taken from the Grado patents that describe operation in some detail. Black is the current circuit, green is the magnetic circuit. Red is mutual inductance path between coils :
Basically, a 'generator ring' steers flux into two coils depending on stylus position, where the sum of the flux in the two coils is constant. Sort of push-pull. Flux in one arm rises as the other falls. Flux direction is always anticlockwise in this diagram.
There are magnetic gaps at front and rear of the coils, and an overall mutual inductance exists between the coils in the magnetic circuit shown in red. This has an associated coupling constant k.
Here's a sketch that shows how externally measured self inductance might arise :
Basically, the sense of the coils and mutual inductance combine to cancel in a ratio (1-k), and overall extrenal inductance in this scheme Lext =(L1+L2)(1-k). 0<k<1. For typical values of k, L is much smaller than (L1+L2).
Here's a sketch that shows how internally generated 'effective cartridge inductance' might arise :
Flux changes in the two coils are always opposing. In combination with opposed winding sense, in this scheme, 'effective inductance' of each coil combines, and the total effective cartridge inductance is L1+L2.
In this way, the cartridge effective inductive is much larger than inductance as can be externally measured directly on the pins.
NB conventional MM/MI cartridges operate differently, and generally such an effect operates with different principle, and the effect appears to be generally reversed.
Obviously, this is just a theory, and is posted here for comment and review. It's also quite simplistic, and ignores all manner of real losses and effects. However, the principle at issue comes through, and since the difference between 'effective' and measured inductance is predicted to be so large, it seems a reasonable approach at this stage.
To hoik this post back to topic, the coil arrangement discussed here is not humbucking. Whereas conventional MM/MI cartridges can be arranged so, the Grado coil arrangement as mooted in this post can't be so. Hence the hum issue, perhaps. So i reckon.