I think you may be misunderstanding what is actually happening. I spent a lot of time researching how profiles affect the routing because the information I had read did not match my own experiences.
But before I go any further, my research was done on a 590, but I suspect that the 39x series behave in a similar way.
What I found is documented in section 4 of the pdf I posted in Post #1 of this thread:
viewtopic.php?f=12&t=521
It is quite complex and I have resorted to colour coding the text in places as the terminology used in Basecamp is not always consistent with the terminology in the Zumo.
But a quick overview.
When you create a route in BC it should be sent exactly as planned to Zumo. If it doesn't then there is something wrong with the maps or the transfer of files. That is covered in the same document.
The only other time that a route gets recalculated is if you deviate, or if you have traffic avoidance tools enabled. Also, if you change the routing preferences once the route is loaded.
A quick sidetrack to Zumo cradles. The Zumo can tell the difference between the cradles- but it doesn't see them as a bike cradle and a car cradle. It just recognises cradle 1 or 2 and it switches some of the preferences to what they were the last time it was in the same cradle. Things like routing preferences, screen orientation and such like. The Zumo can store different settings for different transport modes.
This means that if you set the Zumo up when it is in your hand, and then plug it into a cradle, it may well switch to the other set of preferences ! Switch the routing preferences with a route loaded and the entire route gets recalculated - losing the exact route transferred from Basecamp, but still retaining all of the Via and Shaping points.
Ok, back to the plot.
Very little of the profile information that you set in Basecamp is transferred to the Zumo. Only the profile name - which becomes the transportation mode, and the route preference Eg faster time.
If you have selected a profile that doesn't exist as a transportation mode in the Zumo, then it assumes 'bike'.
And here is the killer blow which results in odd conclusions being drawn about getting the device to work:-
The mode the Zumo is in may change when you plug it into a cradle, and also when a route is loaded. If you aren't looking for it, it is unlikely that you will notice the icon at the top of the screen has changed. It changes back to what it was if you stop the route, but it does not change back when you take it out of the cradle.
eg - The last time your Zumo was plugged into the bike cradle, it was set to use the bike transportation mode and your bike preferences are set as you want them.
You design a route In Basecamp using the car profile.
You plug the Zumo into the bike cradle - the zumo puts itself into the last mode used by that cradle - bike.
You load up your new route designed with the car profile.
The Zumo silently switches to the car transport preferences.
The route loads ok, but now, if you deviate even slightly, it recalculates the current Section of the route using the Zumo's car preferences.
And when you take the Zumo out of the cradle the Zumo remembers that it was in car mode
You have switched the 'bike' cradle to be a 'car' cradle.
If you really want to know what is going on, read that chapter 4. Apologies if the 39x zumos are not quite the same - they are slightly later than the 590. One or two things may have changed.