It does this type of navigation all the time. This was a pretty short "detour" that fit on the screen this time so I took a picture of it. Most of the time it will send me miles out of the way just to re-merge back on the road I was traveling on to begin with... I tried adjusting the routing settings from Easiest, Economy, etc, but they have no effect on this type of behavior.
I bought the F90BT to replace the gadgets I had in my truck (Nuvi GPS & Harmon Karmon Drive N'Play), unfortunately with the F90BTs poor routing, I use the Nuvi for GPS routing and the F90BT just for an overhead view.