The "Educated Register"
Both the sound and letter F was imported from Arabic into Iranian languages, including Persian and Pashto. It is native to neither.
In informal or uneducated Pashto speech, P is frequently substituted for F; thus, Pars (Persia) instead of the "correct" Fars and parangai(Westerner) instead of farangai. (This is analogous with the substitution of B for P in uneducated Arabic speech, i.e., Bebsi instead of Pepsi.) But F is considered "better" and more posh.
However, since the substitution of F for P is a mark of erudition, educated Pashtuns may hypercorrect words naturally containing P. Thus, in hypercorrect speech, the correct pista (pistaccio) may become the incorrect fista.
The same principle applies to qaaf and kaaf. Qs in Pashto are usually pronounced as Ks, but in formal speech the Q will be pronounced in the "posh" Arabic style, with the gulping quality.