Magneto-elastic resonance can cause increased magnetic loss around the point of the resonance. The core A is physically larger than core B, so it resonates at a lower frequency.
After: Thottuvelil J.V., Wilson T.G., Owen, H.A. JR, Unusual high-frequency behaviour of some amorphous metallic-alloy tape-wound magnetic cores, IEEE Transactions on Magnetics, Vol. 20 (4), Jul, 1984, p. 570
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