Why is my page showing as validated in the W3C validator, but not when using SEO checking tools?

I have a web page that returns in W3C Validator as fully validated (and is green).

But when I run a check of the same URL in many SEO testing websites, it fails and tells me the errors it failed on:

Bad value v:url for attribute rel on element a: The string v:url is not a registered keyword.

The relative code this is referring to is schema markup for the breadcrumbs:

<div id="breadcrumbs">
    <span prefix="v: http://rdf.data-vocabulary.org/#">
        <span typeof="v:Breadcrumb"><a href="http://www.bellavou.co.uk" rel="v:url" property="v:title"><span class="fa fa-home"><span class="breadcrumb_first">Home</span></span></a></span>
        <span class="fa fa-angle-right"></span>
        <span typeof="v:Breadcrumb"><a href="http://www.bellavou.co.uk/contact-us/" rel="v:url" property="v:title">Contact Us</a></span>
    </span>
</div>

It's also worth noting that the markup completely validates in Google's Structured Data Testing Tool.

Why is it not validating in the third-party website checks, but more importantly, why is it showing as valid in the actual W3C website?

Solution

Your web page is valid, and that is the reason that the W3C validator tells you that.

But many SEO tools don't understand the prefix in the parent span tag, and therefore expect the value of rel to be one of the typical link types (alternate, nofollow, etc...) and don't accept the one (v:url) that you are using.