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Google Smart Pain in action. Google AdWords accounts sit in different tiers based on a number of factors Google dont tell us about (spend, conversion rate, potential growth.) It sounds like your account got upgraded to the next tier.
We've seen a few accounts where this has happened. Its a clever ploy by Google as if you think your QS is 10/10 then you dont optimise your keyword any further (thinking you have the best QS possible.) Where as you CPC stays the same and if you had a QS of 6/10 then you would spend more time on rewriting ad copy, updating landing page relevancy and load speed time.
By upgrading your QS and keeping your CPC the same, Google actually makes more money as advertisers dont spend time optimising their keywords.
Ignore QS - its a made up number like Page Rank (at best it is a guide.) Look at CPC's, that is all that counts.