why average salary is lower for some graduates

so im trying to pick a insurance uni and i was leaning towards cardiff for the city but on discover uni it shows average earnings for 15 months after the course for biosciences graduates at cardiff university is £23500 whereas warwick it's £28000 so now i can't mak eup my mind ! i know warwick is ranked as a 'better' uni but does anyone know if there is any other reason for this and why warwick graduates earn more?

Warwick is a better and much more desirable uni for candidates therefore its harder to get in which leads to Warwick having better students overall with better grades and with the good teaching at Warwick it leads to them being better developed therefore more attractive to employers so they can get the better grad jobs which ultimately pay more

okay thank you would it make more sense to put warwick as my insurance instead of cardiff if it's the better uni, i know it's ultimately up to me but im being very indecisive sorry ! it is more logical to pick a higher ranked uni like warwick than staying at a nicer city like cardiff even if they're both russell groups ?