Why would 'magnum' primers be any different?
The correct question is why are some makes/models of standard primers different?
The answer to that is historic. The earliest small centrefires using small primers in the 1920s/30s were a) rated at low maximum pressures; b) often used modified rimfire or small centrefire actions. The first popular baby CF was the .22 Hornet and its rifles often used modified RF Martini actions or those from low power CF rifles / cartridges such as the BSA .310 Martini 'Cadet'.
a) meant that the Hornet's and similar cartridges' primers didn't have to cope with >50,000 psi pressures. (.22H [dating from 1930] is rated at 25,000 CUP by SAAMI half that of the contemporary 30-06, and less than half that of the 270 Win that dates from the same period.) So, a thin, weak primer cup was perfectly adequate.
b) meant that many early small CF rifles had weak firing pin strikes compared to the mainly military originating actions used by larger CF cartridges. A thin, weak cup made for more reliable ignition.
The Rem 6 1/2 and CCI-400 conform to the above and date from that period.
In 1950, Remington introduced the first 'modern' small CF cartridge the 222 alongside the M722 rifle, now rated at 50,000 CUP pressure, double that of the Hornet, Bee, 32-20 and similar. It also developed its model 7 1/2 primer to go with it, the first so-called SRP 'magnum', although the explosive pellet of the original version was actually identical to that of the 6 1/2, the differences being in cup materials, construction and strength.
There used to be warnings on some primer cartons and makers' websites warning that the thin-cup / weak models shouldn't be used in modern high-pressure cartridges, particularly 223 Rem with full-house loads.
