The first thing that came to my mind was fly-over or not. Can be quite a big difference on short legs with turns. I understood on your post that this had something to do with it.
Skydemon doesn’t use a great circle route. I observed this when doing very long legs in the US – the route plotted on Skydemon on a leg of a couple of hundred nm would be 3 or 4 nm off to one side the route that the GTN gave at the midpoint of the route. (However, the HSI display of Skydemon did use the great circle route – the HSI display would show you bang on course and in perfect agreement with the GTN, but the line drawn on the chart would be off to one side by 4nm or so mid way through the route).
I have some images somewhere I captured of this, I was going to report it to Skydemon but promptly forgot because flying around the UK, I never have legs between waypoints of hundreds of nm so the issue isn’t apparent).
I have reported this in the past (in my case direct Biggin to Stockholm was out by about eight nm crossing the Norway coast) and was told that it didn’t matter, so I left it. In the grand scheme of things, it doesn’t really matter.
But this event has nothing (or very, very little) to do with that.
Timothy wrote:
and was told that it didn’t matter
Like when they (the US government or something) defined Pi to be 3.2 They soon found out that it did indeed matter, and Pi was re-defined to 3.14 Today we have calculators (or was that 50 years ago? 
) and we get Pi accurate to the 10’th decimal. Are you serious? SD does not use GC calculations?
There have been tons of threads/posts on this in the Skydemon forum. The matter is closed.
Which matter is closed? The inability to plot a GC route (which is curved) or the issue which Timothy is reporting and which is much bigger?
The former.
How generous of them
The bizarre thing about Skydemon isn’t really that it uses a rhumb line, but it uses a rhumb line for planning, but when you actually fly the route it uses great circle instead!
Peter wrote:
the issue which Timothy is reporting and which is much bigger
I don’t think the thing I am finding on the GTN is “wrong” as such. In the FPL catalogue it shows correctly, then, when you fly it, it reads the actual track miles that you will fly, which, I think is also correct.
For example, if you are flying this:
Surely from a time and fuel point of view, you want to be told you are flying four miles, not ten?
