You're referring to Plus-X, right? Plus-X and Tri-X (ASA 125 & 400) are very different to T-Max 100 and T-Max 400 (100 & 400, hardly suprprising).(posted 9880 days ago)In a similar vein, Ilford have FP4+, HP5+, Delta 100 and Delta 400.
The newer emulsions have a very different grain structure, among other differences. Some photographers prefer the new, some the old. So the point of Plus-X is not to provide and "extra" speed between 100 and 400, but is available as Kodak's "old" emulsion at (about) ASA 100.
Strictly speaking, it's called "ISO" rather than "ASA". The American Standards Organisation no longer exists. The International Standards Organisation does.