Abbreviations don’t strictly have to follow any specific logic, they just need to be understood. What you’re describing is an initialism, where you just take the first letters.
My best guess for pfp is that it’s an initialism of the first three syllables because the fourth is unintuitive, three letters was common enough back in the day of BBS and pfp was recognisable enough, while pp is fairly ambiguous. Someone started abbreviating it like this, people picked up on it, and by the time I was old enough to engage with English message boards it was the norm already.
Abbreviations don’t strictly have to follow any specific logic, they just need to be understood. What you’re describing is an initialism, where you just take the first letters.
My best guess for pfp is that it’s an initialism of the first three syllables because the fourth is unintuitive, three letters was common enough back in the day of BBS and pfp was recognisable enough, while pp is fairly ambiguous. Someone started abbreviating it like this, people picked up on it, and by the time I was old enough to engage with English message boards it was the norm already.