Because Kohlschreiber is an extremely talented player, who can challenge anyone when he's 'on', and Murray played very poorly in that match - and because WTA players simply aren't anywhere near the level of a top 100 ATP Pro.
I'll repeat: the rankings bear out that Murray is a top 20 player on clay, or thereabouts. He's not on the level of say, a Ferrer, but you'll remember that he was able to give Ferrer a decent test this year.
But Stosur and Serena are also talented and can challenge anyone on the ATP when they are 'on'. If Murray does not play decently and Stosur/Serena bring their A-game, they can definitely beat him.
And rankings do lie. Pics definitely does not play like he is no.7 in the world
. Similarly, Murray is only ranked top 20 on clay due to his easy draws at the slams and masters series. Highest ranked opponent he beat was no.30, Baghdatis at the French. And he got bagelled in that match too