Leclerc's SF23 was one of the four cars checked for plank wear after the finish and was found to be in breach of the regulations, alongside the Mercedes of George Russell, meaning the Monegasque driver lost his sixth place.
Ferrari was caught out by Austin's bumpy Circuit of the Americas, with the added difficulty of the sprint format preventing teams from making changes to the floor after Friday's single free practice session.
But Leclerc explained that Ferrari saw no wear on Friday night after FP1 and qualifying, so its disqualification came out of the blue.
"Honestly, it was a complete surprise," Leclerc said. "Because on Friday when we could change the car, there was zero wear, so it's not like we were touching anywhere.
"Then you get to the race and obviously, things haven't changed, but we were illegal.
"Rules are rules, and they need to be respected, so it's not an excuse to say that on Friday we were fine. We need to look into it to try and better anticipate what is going to be the wear."
Leclerc revealed Ferrari was even confident of its wear after Saturday's sprint after seeing how little it was bottoming out on the bumpy Austin asphalt.
Source: Autosport