I think it's the depth that can be slightly different Imi. Can you remember what machine you had when you first started to sew?
Most non Janome brand that I've bought, bar some with a purple ish tinge to the plastic, fit my machine. Those that don't fit the bobbin holder or don't perform are used as spares for winding thread for needle thread on my overlocker. I do the same with some very old metal Janome ones.