Given that this was over two years ago, I suspect they've either found a new washer, replaced the tap, or moved house by nowI know this is an old post but as it turns up high on google I'm going to post a response.
The OP asks why is 5/8 washer is actually 22mm. He could have asked why the 3/8 tap washer is actually 5/8. Or he could have asked why a host of other pipework is not actually the size stated.
Why is this? No one knows but it's prevalent in plumbing. My guess is that plumbers don't have rulers so they originally mislabelled parts and no one has ever corrected them.
What is more worrying is that when this valid question is asked, the responses are completely ignorant of the issue and respond with get in a professional. FFS it's a tap washer, he doesn't need a plumber, he just needs an explanation.
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