This GWT graph makes no sense: why does it say my site wasn't indexed til 6 wks ago?

I can’t figure this out for the life of me. My site, jobstr.com, has been online for nearly a year.

But here’s what my Google Webmaster Tools Index Status chart looks like:

This makes no sense to me. Why did Google only start INDEXING my site in early Sept (6 wks ago)?? The green line (“not selected”) has this accompanying definition:

Not selected: Pages that are not indexed because they are substantially similar to other pages, or that have been redirected to another URL.

But I don’t think that would apply to my site…absolutely nothing changed (and the content was as high-quality as ever) before 6 weeks ago(??)

Also unexplained: I’m 99% sure that my site DID show up in Google SERPs (albeit for pretty specific search terms)…so how is it that that graph could be telling me that the site was not indexed by Google, and yet it DID show up in their SERPs?

bump…anyone have ideas? Has been perplexing me, and everyone I ask for weeks.