Man U last year: they still had the FA Cup to play for but they'd won the league and go to the CL semis so weren't really bothered about it. Results before that: 6 wins in 8, 22 goals scored, and only one loss. after that:
Premiership Chelsea 0–0 Manchester United (this one's the killer) Premiership Manchester United 0–1 West Ham United FA Cup Manchester United 0–1 Chelsea
But you're right, they'd never give less than 100% (?!?!???)
It's human nature, if you've been working your ass off for ages and you finally get some credit for it, and you've still got work to do afterwards but it's essentially irrelevant, then it's really hard to motivate yourself for it, that's logical - there's no point being at full throttle your whole life, you'll burn out. We just need the team to learn to turn it on sometimes.