I've been trying to find out as much as I can about peracetic acid, especially regarding its use as a sanitizer. In the [Wikipedia entry](http://en.wikipedia.org/wiki/Peracetic_acid#Production) it notes that 

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> Peracetic acid is always sold in solution as a mixture with acetic acid and hydrogen peroxide to maintain its stability.
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This is definitely born out by commercial examples: Loeffler Lerasept PAA (a sanitizer I've used before) lists 31% hydrogen peroxide, 17% peracetic acid, 16% acetic acid and 1% phosphonic acid among its ingredients.

But, my understanding was that mixing acetic acid and hydrogen peroxide *makes* peracetic acid. From the same Wikipedia article: 

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> It forms upon treatment of acetic acid with hydrogen peroxide.
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So my question is: what exactly keeps the excess hydrogen peroxide from reacting with the excess acetic acid to simply form more peracetic acid? Does the phosphonic acid inhibit this reaction? Is there something else not listed that does this? Or is it just some bit of chemistry I'm not aware of?