Zumo 595 route recalculate after adding stop

[wkc-ps]

[2019-07-01 - Brand new Zumo 595lm]
Newbe here and hopefully just ignorant. Can anyone tell me why, after I add a stop exactly along an existing route (with only a start and end point), when the device recalculates the route it changes it?

This is blowing my mind. I hope there is a logical explanation but, as of now, it seems like a serious defect in the calculation logic. Why would adding a stop along a given route, change the route downstream from the stop?

I noticed this when adding this point to a 10.75 hour ride and it added 30 minutes to the ride! I figured out that it was taking me on a different route between the newly added point and the destination.

Yes, I have triple checked (with an additional pair of eyes) that the new point is precisely along the existing route.

 

[Nuvi-Nebie]

Have you checked that both the 2 point Route and the 3 Point Route are using the same Activity / Custom Profiles ?, e.g :- [Select Route] >> Route Options, all of these options can alter auto-Routing

 

[wkc-ps]

Yes, I am not modifying these settings at all. But I will verify that they do not get modified for me when I simply add a new point...

 

[wkc-ps]

Yes, I am not changing the profile at all. I calculate with a beginning and end and get a route, with a distance and time. Then I add a single point about midway along the calculated route and the route changes. It goes from "508 mi / 10 h 57 min" up to "527 mi / 11 h 17 min."

NOTE - this is with BaseCamp. My initial post here was about doing this on the Zumo 595 itself. But both have similar issues.

Here is what I believe is happening...

The first calculation is from point 1 to point 2. These are widely spaced and algorithms are used to get from point 1 to point 2, using the Routing Preferences.

Then, when adding point 3, roughly midway between points 1 and 2, It performs two separate routing calculations, the first from point 1 to point 3 and the second from point 3 to point 2. Logic dictates that the end result of these two separate routes would exactly coincide with the initial route... but I do not think logic prevails and this simply points out a weakness / limitation in the algorithms being used.

I believe this applies to both BaseCamp and also the code that runs on the GPS device itself.

I find it impossible to trust that the app / device is going to do the job I assumed it would do when I bought it. I find myself with two recourses: 1) return the device, 2) use a different mapping program and then use enough points in BaseCamp to force it to give me the route I really want to take. VEXING!