Just as a guess, "Probably" because this intentional short chambering was not a universal practise by all companies. Many "Experts" were advising against using the longer shells in short chambered guns. The primary concern here was not truly the length of shell but that the newer heavier loads were only put up in 2 3/4" length. Savage obviously was not one of the companies which adhered to the short chamber policy & also obviously considered the Fox guns suitable for handling the 2 3/4" shells so rechambered them & stamped them as such.
It is noted that 2 3/4" shells were readily available prior to the beefed up loads of the 20's, so the fact an older gun has 2 3/4" chambers is not a guarantee it is suitable for modern loads.
