Unexpected results do happen often, but it is more related to team performance. La Liga is one of the biggest competitions in Europe, so the Valencia versus Real Madrid match is certainly difficult to be arranged by bookies, because it is closely monitored by the organizers. On paper, Real Madrid has a fairly high chance of winning the match, because Valencia has performed very badly so far, they are in the relegation zone. So Valencia really needs a miracle to be able to beat Real Madrid.
Ancelotti will use the momentum he has to bring Real Madrid to the top of the standings, while Valencia hopes of getting out of the relegation zone are very low. I think most La Liga fans will see this match as an easy match for Real Madrid to win, and there is no excessive worry about the potential for a surprise.
Honestly, we all know that Real Madrid will win the game, it would have to be exceptionally amazing play from Valencia to see anything else and I do not expect that. As someone who has worked in sports before (not football) and knows the air in the locker room, I know that some teams, when they are not that great, and facing much better, probably champions level of team, they just consider that game as over, and done, and focus on the next game instead.
Valencia players probably think about how they will lose this game but if they could win 2 out of three upcoming games they could do much better, that type of thing does happen within teams and it's not uncommon.
We had that too and we didn't bother, sure the manager/coach would come in and talk to us about how it would be possible to beat them and how we have what it takes to beat them and how there were other bad teams that managed to beat them, and how we could be like all those other bad teams that beat them, but to be fair we never really listen, and we go out and we do our best but we know we would lose so we do not play with our 100% to be fair. This is why I always bet on the favourites, rarely ever on the underdogs.