Was the "coach fare" in this case a Saver fare? If not, do you know what bucket the fare was? The rail fare portion of a sleeper ticket used to be the lowest bucket coach fare, but many months ago (perhaps a few years), Amtrak changed to charging the second-lowest bucket (not counting Saver) fare. If you were charged higher than that, it could indicate that Amtrak has again raised the rail fare portion of a sleeper ticket.

Your example is a bit complicated because it evidently includes at least two trains (presumably the Cardinal and the California Zephyr), and either or both trains could have had either the Saver or lowest-bucket coach fare available. And you are absolutely correct that Amtrak is using higher buckets for the accommodation charge for the sleepers.