Because Andy's played him more, obviously. Do you think Ferrer could improve on his record with Fed if he were given ten more matches? It's doubtful.
The thing people sometimes overlook when considering Andy's record is that most players had a crap awful record against Primerer. The only difference is that Roddick made it to him more. It says something that Miami of this year was the only time they met before a QF.
And consider how he stacked up against Fed in major finals: Agassi, Djokovic, Hewitt, Safin, and Murray have managed to take a grand total of 2 sets in 7 major finals against Federer; Roddick took 4 sets in 4 finals. Only Nadal and Del Po have had better results against Roger in major finals. How many games would Ferrer even manage?
If Andy was a walkover for Fed, a guy like Ferrer is a nap time.
Where was the success in failing?
Roddick had no career, not even in Olympics and Davis Cup versus top 10 guys.
Federer fluked in Wimbledon when Djoker was a mess. In '07 US Open, Fed needed a bad choke from Djoker.
Djoker was hitting peak tennis again in late 2010. Federer never took control of the top 4 opponents the last 3 years, and had trouble beating other top 10 players except Roddick & Ferrer, although he'd love to deny it.
From '05 to '06, Roddick & Hewitt were in finals because Djoker was 18 years old and the tour was a joke. Baghdatis & Andreev couldn't handle Slam chances.
None except Nadal & Djoker won multiple Slams.
Ferrer beat almost everyone for 6 years.
Roddick blamed injury despite losing on every surface for 7 years. He was so afraid, he planned to quit tennis then cried for the cameras in the '09 Wimbledon joke "run".