Yes, but Someday, could it be that with the coming of the Holy Spirit at Pentecost MANY gifts were poured out on all believers. Paul recognises "tongues" as being a gift and having the "gift to interpret tongues" as a gift aswell. PERHAPS this time in history was an explosion of God's spiritual gifts all at once...

Therefore this is the reason why they understood what was being said even though it wasn't their own language. They were amazed as they could interpret and they didn't know how!