AFAIK 48 has always been "the initial number we'll commit to out of the 138", which is very different to "the maximum we'll ever own".
It would not surprise me one bit to discover that the number has been derived against an aspiration to buy additional F35A (or even C) above that total against whatever the FOAS requirement is called these days. A proposal that ought to be staked in the heart at the earliest opportunity to avoid repeating the small fleets / FE@R/ tasking withdrawal cycles that ultimately did for JFH.