Offers at or above the average rate appear, get filled, and disappear. Unrealistic offers below the market rate stay "on marked" for a long, long time - so if you're looking at "what's on offer today" you're seeing a very different list of numbers from the actual prices at which deals are getting closed.
It's same as in stock market - the 'market price' is the average of closed deals, but the visible company job offers is the equivalent of outstanding 'ASK' bids which are not really representative of market price.
You took the job at 15 => Your market rate is 15, all the other companies didn't pay market rate.
Usually, your market rate is the best you got. There aren't many "best" by definition.
The 11 and 12 accepted offers were for other people, that maybe couldn't get better than 11 and 12. Maybe they are less experienced? maybe the 15 company would have hired them both too... but only at 11?
We could say that the cheap companies are under market or that they're only in the market for junior level <profile> (even though they don't advertise it).
We both agree we have to look at the closed deals and the details of the deals =)
Most companies don't pay market rate? That's a tad incoherent? If most companies aren't paying 'market rate' then what should we all call the rate the market is paying?
Hint: what most companies pay is by definition 'market rate'