The rest, he said, was down to the sport's radical new package of regulations this year which included a return of turbocharged engines and electronic braking systems.
Ricciardo, 25, has won twice to climb to third with 131 points after 11 of the year's 19 races, trailing only the Mercedes duo Nico Rosberg of Germany (202) and Briton Lewis Hamilton (191). Vettel, without a win, is sixth on 88.
A graduate, like Vettel, of Red Bull's 'junior' team, Toro Rosso, Ricciardo has out-performed his team mate 6-5 in qualifying and 9-2 in race results.
"It is a combination of several things," Horner told Germany's Auto Bild Motorsport. "First, when you have fought for the title for five years, it does wear you out a little bit, but that is not the fundamental problem.
"The way Sebastian brought out those extra tenths from the car in recent years was quite unique. He is very sensitive to the behaviour of the car, especially when braking."
Horner suggested that Vettel was struggling with the mandatory 'brake-by-wire' braking this year, saying he had "lost part of his feeling for the car."
"The driveability was really bad, so Seb could not look after the tyres in the way that he always has done," he added.
He used to drive a bit "like a ballerina, dancing on the throttle and the brakes," said Horner, a style that was impossible at the start of this season, but was gradually moving back within reach as the team improved the car.
"His pace in Hungary showed he is getting the feeling for the car back again," said Horner.
"And we can't forget how many mechanical problems Sebastian has had - many of them just little things that have disrupted his flow. So he has had less time to adapt his driving style."
Horner said he was optimistic that Red Bull were reducing Mercedes's power and performance advantage and had 'over-achieved' this season.
"Compared to the theoretical handicap we have (due to a 65 bhp disadvantage with their Renault engine), what we have actually achieved in the first half of the year is remarkable," he explained.
"To have won two races, had front row starts, podiums, is a remarkable achievement, all things considered. Obviously, it's not where we are used to being, but the team has shown real character to dig in and fight back."
(Reporting by Tim Collings, editing by Ed Osmond)
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