The reason is that some politicians email systems are incorrectly replying to the "From" address rather than "Reply-To". When we send out advocacy submissions, we use the organization's address as the "From" and put the constituent in the "Reply-To", for example:

From: Joe Constituent <action@organization.org>
Reply-To: Joe Constituent <jcons2007@gmail.com>

That being said, why does it work this way?

If we were to use the constituent's email as the "From", then we would be forging email purporting to be from them. For the legislative offices using Sender Policy Framework as part of their spam filtering strategy, those advocacy responses would get junked.

On balance, it's more important for us to deliver advocacy submissions rather than ensure replies go to the constituent, hence our choice of header configuration.