Year 1979, mister reloader buys some Herc 2400 and a then current Speer #10 manual. There is an asterisk that indicates magnum primers were used with the listed 2400 loads. Mr reloader uses magnum primers as indicated and all goes well.

Year 2008, mister reloader finishes his last blue and silver can of Herc 2400 and buys a bottle of Alliant 2400 and the now current Speer #14 manual. The manual lists standard primers for all 2400 loads. Mr reloader doesn't question the change and uses standard primers and all goes well.
...That is the simplest I can explain.

Now, it is very obvious that 2400 powder has not changed but the reloading data has, can you answer that?
Thanks...