I think we need to see figures from a live system for df -k (if the numbers don't overflow) and df -m and to know what the mount command was for this filesystem. Chances are that it will be correct and that the man df example is wrong.
Mathematically the percentage used should be:
(((Size - Used) x 100) / Size)
However I have seen differences like this caused by error accumulation in Real Arithmetic. i.e. The program was not written to use sufficient precision.
Not on AIX but I have seen a version of "df" which used 1000 as the divisor not 1024 which was scary.
Footnote: Usually this question is because the free space percentage figure is too low! Usually because the account issuing the "df" was not root. This is one of the very few times where I have seen the free space percentage too high.