I answered Software development and engineering when it was on Stack Overflow in 2008. Last night, I edited my answer to that question on Programmers. When I visited Stack Overflow this morning, I had a notification that my answer to the question was migrated. This might have to do with the age of the question and how notifications have changed (for example, I think there was a time when you weren't notified if you answered a question that was migrated), but I'm not entirely sure.
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You were notified because although the question is older it was indeed just migrated yesterday, and that's all that matters for the notification times, regardless of when it was created. The time you edited doesn't factor in, you would have been notified of the migration anyway.