The problem here is that for you consistency isn't one of the aspects that you consider when judging a players "level".
I have no idea why you do that. Results is What matter, and results get you points that get you rankings. It's math.
Answer my hypothetical question then, if Murray plays at #7 level all year long and Wawrinka at #4 who will be ranked higher by the end of the year?
Results do get you rankings, but it's easier to get them when you get easier draws. If Djokovic hadn't started 2010 in top 4 his level of play throughout until the US Open would not have been enough to secure a top 4 position at the end of the year; he'd have got tougher draw and had to face top 10 players, who he couldn't beat for 10 months, earlier in draws. The seeding system in tennis is designed to offer top players staying power, it is ridiculously protective.