What are the reasons for Canadian penny differences as coined since 2003?

In 2003 Canada coined a penny with the prior decades bust image of the Queen in crown. A second minting that year has a more "modern" appearing queen sans crown. There is also a "P" mint mark on the "crowned queen" coin. Later mintings show three variations … some with a P, some plain, and others with a miniature "crown" below the bust image. How come?

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