Royal Mail Redirection does try and match the name used on the redirection form with common alternative versions of you tick the relevant boxes on the application form. I think the example they give on the form is if you apply under the name Susan Smith they'll redirect Sue Smith if you tick the relevant box.
They can't always do this though, not least because the new residents may have a similar name, but also culturally I've knocked on doors doing political canvassing where you have a Muhammed Ahmed, Mohammed Ahmed, Mohamed Ahmed, or something along those lines; and RM can't just assume.
You can always contact RM to query things like this and get small amendments put in place though.
Indeed, this is the main reason I suspect that if a sticker was stuck over an old address without paying extra postage, a surcharge would be made for the recipient. It doesn't help that RM are unofficially prioritising packages over letters and being told to stick more strictly to the rules. Our local postie before we moved was super flexible. I'm betting I could name a few things she used to do which were not allowed but were excellent customer service. I know for a fact she used to do very well on Christmas tips!