why aren't my security codes the same when using Migration Assistance to transfer files from old Mac to new Mac?

Hello funpie,

Welcome to Apple Support Communities. We understand that you’re not seeing the same security code on both Macs. We are happy to help.

We recommend restarting both Macs and starting the process again. Please use the steps in the section titled “Restart” here: Log out, sleep, wake, restart, or shut down your Mac - Apple Support

Let us know if you have further questions.

Take care.