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If the validation within Sigil reports no errors, I will believe that. Did you try the latest version of epubcheck?
The handling of images can differ. ADE uses its own rendering engine and iBooks and Calibre are using a HTML kit to render the books (a different one of course). Since an ePUB is based on XHTML and CSS, it might be a good idea to use a webkit. However, there are some distinctive differences in the ePUB specifications which will be ignored in a webkit. Unfortunatly also ADE has not implemented the complete specifications, but do follow it better.
A lot of companies are lazy (or cheap) and don't want to program a rendering kit specific for ePUB. Hence all the differences in rendering across readers.
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