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What standards? Many companies add their own bells and whistles to the format, which might then be seen as errors by the validators. Hopefully those are made in a way that don't cause harm to readers. So the errors might not be important.
Also, just because an epub is valid does not mean it's error free; there's still typos and formatting errors in them. The series I'm currently reading is like that (Skulduggery Pleasant, bought from Amazon because the epub shoppes refuse to sell outside UK/US).
Fixing those content related issues is more important than insisting on standards, although it'd naturally be best to have both.
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