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 Lewis Hamilton, 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.
"Also on the Saturday night, we could see more or less where we are touching and we thought there was still plenty of margin," he added.
"And then we finished the Sunday and it was a big surprise, so we are still in the analysing part of where exactly did we wear the plank more than what we expected.
Leclerc's race was already compromised after dropping from pole to sixth due to a wrong strategy call.
He felt the lost result was harder to accept because Austin had actually provided more proof that Ferrari's development path was heading in the right direction.
"It's a small step in the right direction, but I think we'll have to wait until next year for the biggest steps. We are speaking about bigger things," he explained.
"I definitely feel more at ease with the car. But that's even more difficult to accept that whenever I have a very good weekend in the USA then I leave the weekend without any points.
"The good thing is that I think we have the proof that I feel more comfortable in the last few races and pushing in that direction should only help me to feel more comfortable."
