"επτά" is older form and sounds a bit more formal than "εφτά".
The old combination "πτ" is phonetically difficult to pronounce, so it has been replaced by "φτ" in modern Greek for phonetic reasons. This replacement exists in other words as well, and not only in επτά - εφτά.
Apart from επτά - εφτά, other numbers have double forms as well.
οκτώ (old) - οχτώ (new) = eight
εννέα (old) - εννιά (new) = nine
δεκαέξι (old) - δεκάξι (new) = sixteen