Relative motion problem

[jmr]

Here's an old post on how to use the in game map as a maneuvering board. I set my map accordingly and plot the two contacts and then perform the final step of vector addition to get the target's true course and speed. My solution matches the example in that post.

However, when I replicate that same problem in Mobo I get a different result.


Here's my plot using the in game map.




And here's the same problem using Mobo. Notice the target's true course and speed are wildly different from the worked example done using the in game map.




Not being the sharpest tool in the shed, can you explain why the results differ?