It's probably largely down to the definition of what a smartphone was back then. The same graph scaled by market size or including dumbphones would be more interesting.
Yeah, I was confused by Windows' apparently-large share as well, and I figured it was a definition thing as well, but if you look at these sales figures:
Microsoft sold 14 million smartphones (or rather, their device partners, probably mostly HTC back then) in 2007, but according to this article from 2007, RIM was running at about 9.6 million devices a year, which matches what wikipedia says:
This graph is kinda confusing to read, but I'm parsing it correctly then it looks like Blackberry had a pretty massive surge around 2009-2010 - well after the release of the iPhone. That is actually surprising to me.
I suppose that might be due to the market growing - in which case Apple looks like it's relatively flat starting around 2010 but in reality the number of users were increasing along with the market.