JSON is slightly worse than you describe - the JSON language doesn’t restrict you to 64bit floats, but most implementations do as you describe. On the other hand, the JSON language doesn’t support NaNs or infinities, so the union of language and implementation means that in practice JSON is strictly weaker than IEEE 754.
If you count by threes, the ones place is unique until you pass each multiple of 30 - 3, 6, 9, _8, _1, _4, where was I? I hadn’t made it to 30, ends with a 4, must be 24. 27, 30, 33, _6…
Serves as a sort of checksum, as long as you know roughly how many you have and just the last digit.