Here's some doubles to look at
Froome
2012: Dominant Tour win 2nd place - fades in Vuelta and gets 4th at like 10 minutes.
2015: Dominant climbing level along with Quintana in the Tour - was getting dropped by 15 guy groups early in the Vuelta
2016: More moderate win in the Tour, got 2nd in Vuelta and definitely 2nd best climber
2017: Tamest Tour win, but wins Vuelta pretty comfortably
Quintana
2015: Highest Tour level - significantly weaker in the Vuelta
2016: Underwhelming Tour - Wins Vuelta
2018-2019: Kinda meh in both
Valverde
2008: Inconsistent in both Tour and Vuelta
2012: Bad in the Tour, one of his best Vueltas
2013: Good in Tour apart from crosswinds, good in Vuelta, but neither were his best career performances in those races
2014: Bad in Tour, better in Vuelta
2015: Best Tour, weak Vuelta
2016: Let's ignore cause Giro
2018: Bad Tour, slightly solid Vuelta
2019: Bad Tour, very strong Vuelta
2020: Equally washed at both Tour and Vuelta really
2021: Terrible at Tour, but was looking better in Vuelta
Purito
2010: Probably similar level in both Tour and Vuelta
2013: 3rd in Tour and 4th in Vuelta, with the former being clearly higher level
2014: Tries to KoM Tour, much better in Vuelta
2015: Stage hunting/KoM Tour, gets 2nd in Vuelta
Mas
2020: Only consistent double, 5th in both
2021-2022: Very meh at Tour, best of the rest at Vuelta
This pattern basically holds for a lot of other riders as well. Nobody holds the same level in the Vuelta if they're flying to the moon in July