I see your point, technically he was closer than the other two. What I meant is that he wasn't affected by some unexpected/extraordinary factors, but was rather outplayed solely by his opponents.
Sampras had a monstrous season in 1994 until he got injured, was at his physical peak and in the form of his life, winning AO, the Sunshine Double, Rome, Wimbledon and several other titles. Plus he was the defending champion at the USO, so it's hard to see him losing this title - Federer never experienced anything similar (imagine him not being able to compete well enough at the USO 2005).
In 1999, he was playing some of his best tennis during that summer, beating Agassi (#1 player in the world) left and right, and run at Cincinnati was particularly impressive, scoring wins over Krajicek, Agassi and Rafter (in a row) without losing a set to win the title, but then had to withdraw.
Regarding Connors, he was by far the best player on hard court on Tour since hitting his prime (1974) and had won the biggest titles on the surface more than any other player, twice at Caesars Palace (Las Vegas), twice at Washington, twice at Los Angeles and of course the first edition of the USO played on HC. It's fairly reasonable to assume that he would've won another title between 1975 and 1978. HC was his best surface anyway, suited his big, power game with a great return pretty well.