The Itch price in £ I'm quoting is simply the converted rate. As offered by PayPal in the UK when going through the checkout.
Steam allows the developer to set regional prices - and presents the cost as per regional expectation (tax excluded in US, included in Europe). This is why the USD price is the same on Itch v Steam.
I assure you the UK final price is different, by some way, I have been through the checkout for both. I have seen this for a number of different games, usually smaller indie games (A Short Hike, Baba Is You). All I can think is when the developer has set the UK price on Steam they have not realised that tax is included in the upfront price, hence the final price being a fair amount higher on Itch.
On that steamdb page you linked it shows the UK price *including* tax versus the US price *excluding* tax. That doesn't seem like a fair comparison.