Different way´s to get the error output from a previous activity?

I have a pipeline executing another pipeline with an execute pipeline activty. The error output is connected to a second activity with a sp where i log the output of the first activitry in a database.

By trial and error it seems there are several ways to capture the output

• to get the error message from DataFlow Activity: @activity('DataFlow').error.Message
• to get the error message from CopyActivity: @activity('CopyData').output.errors[0].Message

Why the difference in the expression builder?