[hakunabatata]

Is this a reliable method of testing the purity of a chemical compound - (KIO3)?
How can something be more pure than 100%?

We have three - KIO3 compounds (every from other manufacturer), all in the same class ACS, e.g. 99.7-100.4
According to CoA - one - 99.8%, second - 100.1% third - 100.4%.
Specified impurities (heavy metals and so on) are at virtually the same level.
Only diffrences is pH.

Does this always mean that physically this 100.4 is the purest compound, with the least amount of residue?
Is it possible that, for example, a compound that has 99.8, may turn out to be purer than the others, therefore safer?

[marquis]

First step is to check the references.  Is it an astm method?  Do a little more digging, if possible, in any reference publications. 

It's possible that this is within astm limits.  Sometimes, the variability in test results gives readings over 100.0 %.  Especially when you apply 2 or 3 sigma deviations to the answers.  Or, it could be indicator error.  Whatever the cause, it would be a good idea to know what the source is.  Someone else will probably be asking the same question you are.