The former champions are now getting closer to title protagonists Mercedes and Ferrari, but only after a hard push to identify the problem and close the gap.
"The race track didn't show what our tools had promised," team boss Horner told Auto Motor und Sport.
"It put us back two months, but then you have to work twice as fast as the others because they're not standing still."
Asked what exactly went wrong at Red Bull's Milton Keynes headquarters, Horner explained: "It was mainly the wind tunnel that let us down.
"The larger model and tyres gave us different results between the track, the CFD and the wind tunnel."
However, another issue has been customer engine supplier Renault, Horner said.
Asked how much the French-made engine has improved since the start of the season, he answered: "I'd say two tenths.
"By Renault's own estimates they are behind their targets, but they are working very hard to catch up."
However, other factors may also have been in play, like Ferrari's supposed 'oil in fuel' technology trick, and the fact that a clever Red Bull suspension system was banned.
Referring to the oil trick, Horner said: "It definitely benefited those who had it, but it wasn't Renault. Now we're all back on the same page."
As for the suspension, he added: "We tested the system but it didn't give us enough to justify the additional weight, so I don't think that hurt us at all."
Red Bull are currently third in the constructors' standings, but some 134 points adrift of second-placed Ferrari.