Variants in the Mushaf of Uthman?

How can Qiraat arise if the majority of the Quran was written in the Quraishi Dialect? I previously asked the same question but no reasonable answer. I might have misunderstood the meaning of the Hadeeth: **"If you disagree with Zaid bin Thabit at any point, write in the dialect of Quryaish. Verily, the Quran was revealed in their tongue."** Please, explain it to me, I really dont understand? Also, do we know that Uthman purposely allowed these differences?