Your personal interpretation won't make a difference if the rangers, well within their rights, choose to enforce proper permit usage.
Imagine you started hiking the first day permitted on your permit and you got back NEAR the trailhead 5 minutes before midnight. A ranger happens to stop you and sees you have an overnight permit. I highly doubt he would make you pitch a tent and sleep for 10 minutes before continuing to exit the trailhead the "next day".
That's not a useful hypothetical scenario. The likely scenario is if you get permit checked during the day and your permit or tags show an overnight permit, but you are not carrying overnight gear. The lack of overnight gear would reveal that you are not overnighting. The ranger could then choose to fine you and escort you out. Whether or not they do that for misusing overnight permits, I don't know.
