Ferrite 604 high FS HF diaphragm?

[gearfreak]

In preparation to post my 604's for sale, I've DATS tested the 'free air parameters' (*with loading cap, on the matnray horn) of both HF sections.
One looks normal (fs ~410hz), the other comes in at fs=1420hz!
Re and Le are comparatively the same.

Looking at the impedance plots, they track similarly in both areas:
broad peak around 400hz
sharp peak around 1450 hz

...so not sure why DATS has decided the blue one has such a high fs. Am I looking at a diaphragm problem, or is DATS wacky?

I figured the 1500hz was the horn loading up (?).

pic attached fingers x'd