Idaho license fees - what am I missing?

[dubob]

So, as I read this, I can purchase a three-year license, before Dec. 1st, and of course I would need to pay for three years and it will only be valid for two years!  Under what circumstances does that make sense?  What am I missing?

The circumstance under which this makes sense is the one where you purchased this 3-year license in December of 2019.  Just saying.  If you haven't already purchased that license, then it would be financially unwise to buy it now.  But I'm pretty sure you already know that.