Trip optimisation

Requesting a trip from Bordeaux to Rennes display a recommended travel thru A10 - A83 (near Niort) then Nantes and Rennes. Also an alternate one is proposed taking the exit 7 at Saintes and going toward Rochefort, La Rochelle and Luçon to retrieve the A83. The road is straight forward and shortest in Km.

Why the opposite trip does not (never) shows the shortest path (requesting a Rennes to Bdx travel) and always passing Niort ?

Thanks for the clarification or which criteria I forgot ?

Regads