P swapping places with F

There's an interesting phenomenon I've noticed in Greek and between two Slavic languages. The Greek word for minute (time) or cent can be spelled λεφτό or λεπτό (leftó or leptó). There are also Modern Greek εφτά (eftá)and επτά (eptá), seven, from Ancient Greek ἑπτά. Another case is Ancient Greek πτερόv (pterón), Modern Greek φτερό (fteró), meaning feather, wing (that's where we got our word pterodactyl). Somehow related to that are Czech pták and Slovak vták (pronounced ftahk), meaning bird. Maybe the Ancient Greek φ, which some scholars believe was pronounced ph, not f in the classical period, and later became, has something to do with this, but how the same phenomenon occurs between two Slavic languages beats me. It looks as if the biggest culprit here is the cluster pt or ft.