Actually it's far easier to fluke a #1 ranking than a Slam. In order to win a Slam, you have to win seven best of five matches against the best players in the world, you can't BS your way.
Meanwhile, the ATP ranking system isn't very different from the WTA one that generated some truly abhorrent #1s. You can easily be #1 without being the best player in the world as it is far from an objective/reliable measure; you can't BS your way to a Slam title though.
Lenders, Lenders, Lenders... Let's start off:
1. You DO NOT always have to play the best players to win a GS... For example, Lleyton Hewitt Wimbledon 2002
R1: Unseeded 30 year old Jonas Bjorkman
R2: Qualifier Gregory Carraz
R3: Unseeded Julian Knowle
R4: Unseeded Mikhail Youzhny
QF: 18th seed Sjeng Schalken
SF: 4th seed Tim Henman (bad record in SF's)
F: 28th seed David Nalbandian
Did Hewitt have to beat the best players to win that? Nope.
2. While the ranking system might not always provide the best player in the world, you have to win a string of matches/tournaments to be ranked at #1 (far more than 7 matches when someone is on a hot streak) It is far more likely to fluke a GS then No. 1 and that is a fact.