Now....you gotta wonder why the differing numbers. I mean, it's for ease of use rather than a non-repeating decimal, sure...but why choose THOSE numbers?
6300 is close so that's good
6000 is easily used, so that makes sense
Why 6400?
Computers, especially the older ones are binary. Zeros and Ones. (Actually, on the first computers, those represented switches in states of being open or closed). Binary (and every base, be it base 10 or 12 or 16 or whatever) are written as powers of the base. Base 10...the number 10 is 10 raised to the first power. 20 is 2 times 10 to the first power. The hundreds place is 10 to the second power.
2^6 is 64, nice for the computer iteration of a firing solution for artillery.