Yeah, they could have constrained the random value to always start with the highest bit set to 0. Then they would be guaranteed to be able to generate 2^79 valu

Yeah, they could have constrained the random value to always start with the highest bit set to 0. Then they would be guaranteed to be able to generate 2^79 values per millisecond. But that's sort of an implementation detail. You could write an implementation that did that and still be compatible with all other implementations.