Toyota charging system

I put a voltmeter on a  2017 Highlander that I just bought. The readings don’t make any sense.

When it is idling in park, the voltage is 14.6V. If I am idling in Drive, it is 12.4V. RPM ~ 700. If  I give it a little gas to bring the RPMs back to 1000 it does not change back to 14.6V where it was in park. When I am driving, the voltage is 12.4V. RPM can be anything. If I take my foot off the gas pedal while I am driving, it goes back to 14.6V. That can't be right.

I expect it to be in the 14V range while driving and possibly to decrease if the RPMs fell as in when I took my foot off the gas.

I have checked the belt and it is in good condition and tight. There is no noise that would indicate it is slipping.

12.4V is not a fully charged battery. So why is it reading 12.4V at higher RPM when driving and increases to 14.6V when I take my foot off the gas and RPMs drop?