Premium domains are classified as premium by registrys. A registry owns a TLD, such as .love[1], and tells the registrars (Namecheap, GoDaddy, Cloudflare, etc) that it's premium either when they try and register it, or they keep an updated text file of them usually. ICANN decides which registrys own which TLDs.
When Namecheap tried to register your domain, they were told by Merchant Law Group LLP[2] that the domain you (the registrant) want is actually premium, and the real cost of it is $1,950. I assume Namecheap simply used the current normal price, and when they find out it's premium, they update the cost.
[1] https://icannwiki.org/.love
[2] https://icannwiki.org/Merchant_Law_Group_LLP
While we're on the topic, it might interest you to know that ICANN actually has regulations preventing registrys, such as Verisign (.com, .net, etc) and Merchant Law Group LLP (.love), from changing the price of renewal from normal to premium. This is to prevent something like, a registry letting a registrar let you buy your TLD for $7.99, but then the registry arbitrarily tells the registrar "the domain is now premium and the renewal cost is $2,000."
Normally this would be prevented via ICANN regulations (that registrys agree to, otherwise they lose their TLDs), except there is a stipulation. The registrant (that's you, or whoever is buying the domain), can voluntarily forgo and surrender these rights in a written statement. Coincidentally, all the registrars (Namecheap, GoDaddy, etc) have this written stipulation in their contracts, that you must to agree to in order to be able to purchase a domain. This means that registrys can arbitrarily change an already registered domains status from non-premium to premium.
The real question is why do the registrars do this for the registrys? They are supposed to be separate companies and entities entirely and prevent things like this from happening. I imagine it's because registrys can pick and choose which registrars are able to sell their TLD. So if a registrar refuses to add this in their agreement with you, then the registry simply denies the TLD request by the registrar.