At the time I attributed it to the cone assembly. Out of the LE10H, 128H, LE14H, 2235H and 2245H only the 2235H used a mass ring instead of aquaplas. If I remember correctly, of the LE14H, 2235H and 2245H, only their cone assemblies (including three different geometries) and frames varied, the voice coils and magnetic return assembly were equivalent. There was a time when I cared enough to remember and could have answered definitively.
I believe the same held true with the LE10H and 128H, only the cone assemblies and frames varied. I do know that JBL added or subtracted a few coil turns from both the LE10H and 128H at some point to achieve whatever changes they were looking for.
In any case, transducers such as the 1501Fe, 1501AL, and 2216Nd really are
"light years better". And that doesn't have to be a problem.