Using GoEuro, if you type in Frankfurt, the pulldown doesn't show the airport, just the Hbf. He probably got the Hbf by default. OnlyTrains shows the airport, but it is down the list; Hbf is at the top.
I can think of no reason to use GoEuro or OnlyTrains or any other 3rd party site instead of the German Rail website. It's German Rail that creates the tickets; other sites are just resellers and copy the information from German Rail.
All 34,90 Euro
All? Who shows 34,90€? The actual ticket price shown on the Bahn and TrainsOnly is 29,90€. TrainsOnly shows 35,90€ because they want to sell you a seat reservation (4,50€ on DB) for 6€, a profit for them. I don't know about GoEuro because they would only show me the schedule if I downloaded their software, which I would not do. It probably includes some kind of tracking software, or worse.