For one, those measurements can't possibly have been recorded by Plato or a contemporary, because if there were a historical Atlantis all claims point to it being centuries older than Plato. Any numbers he's using are either completely fictitious or passed down and distorted through generations. Either way, Plato clearly didn't care that they were accurate.

Second, ancient literature is kind of obsessed with giving precise measurements for things that can't possibly be accurate. The numbers are inserted to give a sense of grandeur, and are just a part of the style of this kind of work. Where they seem odd and out of place to us, they were common at the time. See the Bible, Herodotus, and many others.