why does my zumo 396 calculate a route that's been calculated by Base Camp?

I'm sure this is part of the issue. However, there is a basic compatibility issue between BC and the XT2 that Garmin makes no excuses for. They do not support or recommend using any mapping application for the XT2 other than Tread. In their words,"the tread app was designed for only the Zumo XT2 and Tread devices." That said, there are numerous posts about issues between the Tread app and the XT2, too.

Firstly: BC and any device calculate differently.

Secondly: use tracks instead of routes if you want to be sure to follow your preplanned drive.

Firstly: Agreed.

Secondly: I tried that. The XT2 converted the track to a route and in doing so changed it more than if I had sent a route from BC. I you have had a better experience sending tracks from BC to the XT2, could you, perhaps, post some tips? I've read others who have had good luck sending tracks from BC to an XT, but not to the XT2. Thanks.

I have an XT.  I created my routes in BaseCamp.  About 50% of those routes calculate when imported into the XT.  There are a lot of suppositions as to why and I have addressed them as I see them but I have yet to resolve the calculation of some of the routes on import.  I now create a track of each route in BaseCamp and transfer them both to the XT.  I then immediately import and then compare the two by showing the track on the map and then loading the route.   Any variances are usually easy to see.  It is a pain to have to do this but it is the only way that I can feel certain that the route is as intended when it comes time to ride.

AFAIK, some devices, like the XT and XT2, 'learn' your riding style and road preferences.  Basecamp does not.  This could explain why the XT calculates on import, especially on long routes or routes with long distance between route points.  It is trying to 'help', as if we aren't capable of specifying the roads we want to ride on.  It reminds me of my new car.  I took it to the dealer out of frustration in how it drives only to be told that it is responding to how I drive.  This while I'm trying to adjust to how it drives.  The car is a machine.  It shouldn't adapt, I should.  The XT is a machine.  It shouldn't adapt either.  If we tell it what to do it should do it.  If we ask it to get us to a point it is then free to use its brain within the parameters configured (eg: Faster, Shorter, avoidances,...).

Thank you. I misunderstood your previous post and thought that you were converting the BC track in the XT. I tried that in the xt2 and it was a mess. Now, I understand that you import both a track and a route and compare them. Presumably, the track shows exactly where you want to go. If the route varies from that, you can amend the route in the XT/XT2.  As you say, it's a pain, but it seems unavoidable. In reference to the xt2 learning my riding style, the Garmin engineer refers to this as it being very accurate compared to BC and Garmin Explore. I prefer your characteration. I don't want the damn thing to tell me where it wants me to go. I want it to guide me to go where I told it I want to go. That seems to be too much to ask.

I do not have an XT2. But I have experience with quite a row of Garmin devices, like iQue, zu.o550, nuvi3598, edge 705, edge800 and edge1030. They all worked more or less the same apart from the nuvi where you can create trips only on the device.

So the behaviour you describe is surprising to me and I can't give you better advice.