Why Does ART Compile Apps After Upgrade?

Question

I have a Google Nexus 5. Every time I do a system update, I see this message:

Android is upgrading...

Optimizing app [x] of [y]

I understand that ART is compiling my apps. What I don't understand is why. What happened to the previous compilations?

Answer

Because the ART compiler can get better each version after your OS gets upgraded. If Android makes any improvements to ART including bug fixes, the apps will need to go through the same processes as the compile part of the installation.

— Morrison Chang | Sources

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