Are US units correct?

[Joe Horn]

According to the HP 50g's unit conversion utility, these are true:

1 chain = 4 rods = 66 ftUS
1 fath (fathom) = 6 ftUS

But Prime gives these conversions:

1 chain = 4 rods = 65.9999999982 ftUS
1 fath = 5.99999999984 ftUS

Perhaps it's all based on this difference:
1 meter = 3.28083333333 ftUS (HP 50g)
1 meter = 3.28083333324 ftUS (HP Prime)

Which is correct?

EDIT: Something is clearly amiss in the Prime. It says:
1 miUS = 5279.99999983 ftUS
But in actuality 1 miUS = exactly 5280 ftUS. It looks like Prime's definition of ftUS is screwed up.