Edit: Your 50A limit is due to your choice to use a 50A socket instead of hardwiring, which you say flows from belief that this will allow you to plug your EVSE into solar panels during outages. You'll want to carefully fact-check that belief. I cannot emphasize this enough.
All continuous loads are derated to 125% (40A load needs 50A breaker) because consumer tier gear is just not rated for continuous use. Certain loads are always treated as continuous loads, including EVSEs per NEC 625.42.
NEC 210.19(A)(1) General. (a) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load in accordance with 310.14.
There is an exception for industrial gear which is rated for continuous 100% rating, however that is very costly and a 25% larger breaker is a lot cheaper.
There are no other derates, unless you are dealing with thermal derates due to environment.
The normal approach is to use 50A wire to a 50A socket to an EVSE commissioned for a 50A circuit. The EVSE tells the EV to take 40A, which x 125% = the 50A given in the circuit.
Of course there is nothing wrong with using larger wire than is required, but such optional behavior is not a derate.
I have no idea why your installer used a 50A breaker with 65A wire. But I can make a few guesses.
First, the most popular cable for hooking up 50A EVSE's is 6 AWG copper NM-B or UF-B. That is not 65A wire. That is 55A wire per NEC 334.80 and Table 310.15(B)(16) which does not have any 60A wires in it. For reasons which seem utterly bizarre to me, EVSEs have a 60A setting but don't have a 55A setting. Clearly, nobody looked at "El NEC" when designing EVSEs.
Second, it may be an arbitrary limit of your EVSE. EVSE's are only rated for so much current internally. Handling more requires a larger contactor and heavier wires both internally and in the costly J1772 cord.
Third, perhaps someone has weirdly specced this thing to be cord and plug connected (why?), and the NEMA 6-50 or 14-50 socket maxes out at 50A per NEC 210.21. So that is a code requirement.
Fourth, perhaps your EV, wiring and EVSE may all be ready for 65A, but your house's service is not. Really, that should be the first thing to check. You can't just keep dogpiling on more and more load onto a house's electric service - A/C, hot tub, tankless water heat, large EVSE, compressor, yadayada. There are rules for determining what can fit on a service called a Load Calculation. So your installer may have properly done a Load Calc and determined 50A is all that can be safely added.
Fifth, it might have been a long circuit and larger wire was warranted for voltage drop reasons.
Or sixth, the installer might have been a novice who simply conformed with random Web (or worse: Youtube) advice, saw "everybody" using 6/3 NM to a 14-50 socket, and as a result selected #6 THHN when #8 THHN is good enough. To that I cite Harper's Rule: Buy the wire last. Implying one should seek advice from the community and let all information be gathered before making purchase commitments, especially for non-returnable wire!
Seventh could be dealing with a thermal derate either due to hot spaces (Arizona attic) or multiple circuits sharing a conduit longer than 2 feet. For 2-3 circuits in a conduit, #6Cu derates to 60A. For 4 circuits it derates to 52.5A. For 5-10 circuits it derates to 37.5A.
Editorial time. People often think they need way bigger charge points then they actually need. A 240V/20A is enough for almost everyone almost all the time - 100-120 miles in a 10 hour charge session. And for those "but Sometimes..." moments, there is DC fast charging becoming exponentially more available.
As such, the 65A capacity wire puts you in a good position if you get a second EV. Now you can use Share2 technology EVSEs to share the 65A among two different cars, splitting dynamically - even if both cars charge all night (unlikely) they'll each get 32A (provisioned; actual 25.6A). When one is charging it gets system limit, probably 60A (provisioned; actual 48A).
