Your OP wasn't clear that the pricing difference you found was on th same route. As I am sure you know, the floor varies based on origination and destination. LAX-HNL floor is different than LAX-ATL floor.
R should be the lowest and the fact that you saw two different prices for R is a bit concerning. The more details you can provide the better we can come up with a theory.
We're both options non-stop? Were there partners involved or all on DL metal? Both options only displayed the fare code R (not RL or RD), right?
One of the flights is priced at 40000 miles in business, ... Another one of the flights in the same day is priced at 50000 miles. they are both coded "R" fare.