Sebastian Vettel had gone into the 2013 Formula 1 season opener as clear favourite, having shown impressive pace in practice and qualified on pole position.
However, the world champion was not able to look after his tyres as well as his rivals and could manage no better than third as Kimi Raikkonen and Lotus won.
Horner reckoned that the team's race set-up choice was incorrect for the Melbourne temperature.
When asked by AUTOSPORT for an explanation about where the team's early weekend form had gone, Horner said: "One simple word: tyres.
"We just weren't in the optimum window with the tyres today, for whatever reason. For the conditions and so on we were a bit out.
"That became fairly apparent from lap four or five, because Seb got a good start, he built up the lead that we wanted and then it was quite clear that the car was pretty heavy on the tyre.
"That forced us then to go down the route of a three-stop and, when you run into a bit of traffic as well, that puts more stress on the tyre."
Horner did not think that the tyre management issues were an inherent weakness of the RB9 but instead were simply related to the temperature.
"I think probably the conditions were a significant factor today and we were just out of the window," added Horner. "All things considered, third place was actually a very, very good result.
"Kimi was the only driver/car combination that could make a two-stop really work and it was quite obvious from half distance that is what they were doing.
"To emphasise that point, he did the fastest lap on a tyre that was older than we could have dreamt going anywhere near.
"So I think we have learned an awful lot today and hopefully it will not be quite as chilly next weekend."
AUTOSPORT Australian GP coverage:
FP1 FP2 FP3 Qualifying Race Gallery Live commentary Official quotes
- Sports & Recreation
- Motor Racing
- Kimi Raikkonen