Apostrophes in keywords

[MulberrySqCraig]

It appears that SEOmoz tools like the On-Page Report Card treat keywords differently if they have apostrophes in.

So for example childrens and children's are treated as different keywords.

Is this a quirk of the On-Page Report Card or does Google treat them as separate keywords?

[RyanKent]

Any variation of a keyword can be treated differently by search engines.

Try searching "children's books" and "childrens books" in Google.com. The first 6 results are identical but after that point you will notice the results change. Depending on the exact phrase used, the first result can change.

[MulberrySqCraig]

This does seem to post a problem for SEO. Obviously on site everything wants to be spelt and punctuated properly, anything else looks amateurish. But when searching a majority of people don't bother with punctuation and are much more likely to just bash in childrens rather than children's, which means that sites who optimise for childrens will be ranked higher then those who punctuate words properly.

I'd give a comparison on how many people use apostrophes when searching but infuriatingly Google Insights doesn't take apostrophes into account.