This is a matter of curiosity, as this has now happened twice. We flew Frankfurt to Philadelphia last July and this week. Both times, although there was certainly adequate room at the gate, the plane was not there and all had to go down several levels of stairs to then take buses and board in the parking lot (and to make this even dumber, the plane was way west and had to taxi across the airport to the east end). For some of the passengers, this was significant extra physical effort.
Since Frankfurt is essentially Lufthansa, does anyone have an idea why this flight is not at the gate?