Why did the 40th-year Jews think they'd erred about Av? Sanhedrin had declared it

Question

Rashi says that

ובשנת ארבעים שנה עשו ולמחר עמדו כולן חיים וכיון שראו כך תמהו ואמרו שמא טעינו בחשבון החדש חזרו ושכבו בקבריהן בלילות עד ליל חמשה עשר וכיון שראו שנתמלאה הלבנה בט"ו ולא מת אחד מהם ידעו שחשבון חדש מכוון וכבר מ' שנה של גזרה נשלמו קבעו אותו הדור לאותו היום יו"ט

... On the fortieth year they did so [- dug graves for themselves -] and the next morning they came out alive. They were wondered "Maybe we made a mistake in us calculating the month?" They went back and laid in the grave until the 15th. Once they saw that the moon is full (on the 15th) and no one died, they knew that the calculation was right and the decree [that all Jews who were 20-60 at the time of the spies must die in the desert] was finished...

In other words, they thought that the 9th of Av was a miscalculation (and it should have been on another day). However, it says in Rosh Hashana that

הרי הוא אומר אתם אתם אתם ג' פעמים אתם אפילו שוגגין אתם אפילו מזידין אתם אפילו מוטעין

It says "You" three times: once even if one makes a mistake, once even if one did it on purpose and once even if they were tricked.

---

So if Kiddush Hachodesh makes the month fall out when the Sanhedrin said it did, what were the Jews worried about that they went back into the graves for another 6 nights.

Answer 1

Rabbi Yonosan Eibseshitz address this question in Yaaros Dvash (Chelek 2 Drush 4). As far as I can understand, the simple explanation why a mistaken sanctification by Beis Din was not binding in this case is because (as the Gemora in Rosh Hashana 20a explains) this is not applicable in a case of "מיחזי כשיקרא"; if it appears incorrect to the masses Beis Din does not have the power to manipulate the calendar per their wishes. However, he asks how it was possible in the first place to make such a mistake of five days, especially considering that they could have counted two weeks from the last full moon to have an idea when Rosh Chodesh would be. He explains that during Aaron HaCohen's lifetime they had no way of seeing the moon to sanctify because of the Clouds of Glory. Therefore they had no choice but to rely on the rule that even if the calculation is incorrect, the Beis Din's sanctification is binding. This is not considered "מיחזי כשיקרא" because no one would be able to see if they wrong. Ahron HaCohen passed away on Rosh Chodesh Av of the 40th year so that was the first date that they could santify the months based on sighting the moon and were concerned that their calculation may be off by a few days.