This is not just an issue with MRA, it's a generic thing.
Back in my Garmin days, I had several encounters where I created a route in Garmin's Basecamp software, then send it to my Garmin Zumo device. When Navigating the route, it was sometimes different than expected.
The reason is not just different algorithms used in the planning software and the device, also different parameters.
You can memorize it like this: a route you create is just a list of via points. Your device will recalculate a route, from point to point, along with its parameters, like Fastest route, shortest route, avoid gravel roads, avoid blocked roads etc.
As someone else said, add more waypoints to your route, then you don't have to pay much attention to parameters.
