The existing cubit measuring rods vary a bit in their lengths, and each architect had his own set of tools (buried with him.) There's many fine examples of them (photographs available online) which shows that they are about as accurate as the carpenters' wooden rulers (measuring sticks) made by other civilizations.

In other words, there's a limit to their precision and there's a range of lengths which can vary up to several millimeters. They couldn't make multiple-decimal-point accuracies and they couldn't replicate multiple-decimal-point accuracies -- not deliberately (they couldn't cut rock THAT precisely, even if they had by chance some means of measuring it down to the level of microns.)

Their tools weren't capable of that.

-- Byrd
Moderator, Hall of Ma'at