Why aren't nel coils interchangeable?

[Pa.]

Garrett make their coils interchangeable in the AT series and for other series in the product line.
So if I buy a coil for the AT PRO it can be used on an AT MAX or AT GOLD.
Yet if I buy a nel coil for the AT PRO nel say it won't work on the MAX or GOLD even though it's the same connection. I know they all run on different frequencies but if Garrett and other companies can make a universal coil why can nel ?