Did you answer your own question here? After you hooked the correct (matched) sender to your aftermarket gauge you get a more reasonable, and, theoretically more accurate, reading.
You might have had two temp senders on the top rear of your cylinder head, one for the dash gauge and one for the emissions control. they each put out different amounts of whatever it is that they put out (I don't know much about electricity). The markings on the sender might tell you what that sender was sending if you could read that code. Figure out which sender does what and you can run your stock temp gauge and your new temp gauge. That's what I do.
I like gauges, I like mechanical gauges better than electric gauges.
View attachment 3784982
View attachment 3784984