This is driving me nuts! 
I have a very basic shape
I want to straighten the UV. But when I select a pair of points, there’s always a third mystery point included
Why is this!?
[ I’ve only unwrapped the top face. There are no overlapping verts ]
Thanks 
For 3D vertices all UV-vertices are selected… to handle this differently: split you model or in
in 3d-view select all veritces and then disable UV-sunx (top left in UVeditor)…
DOH! - Thank you so much - that’s several hours of my life I’ll never get back… 
There is not one second you could get back… but you learnt something today 
and next time maybe just have a different look at such a problem… ( … time for a pause… well there are so many buttons… let’s try one 
)
There should be a warning on that sync button
I always thought it was a good idea to keep it on, very handy. NOT when it comes to follow active quads…
Maybe it is easier to just use follow active quads to unwrap the object instead of straightening the uv’s after unwrapping.
Select the faces to unwrap leaving one as active, press U to unwrap and select reset.
Then press U again and select follow active qauds.
You will have to do this separately for each side of the object, and compensate the aspect ratio scaling the result.
For this shape you could do it with 2 selections.
Selection 1 (after “U-reset” + “U folow active quads”
Selection 2
Result after compensating scale and moving islands
FAB!
I didn’t know you could do it that way, great
Blender is not easy to get to know, I must say. I think it’s because it’s had such a multifaceted upbringing…
Yes there are soo many options, I have been using Blender for over 10 years and have not used half of them yet! 
There’s also a faster way to align - select an edge in the UV Editor, press F3, type in Align, right click on Align X and Align Y and choose Add to Quick Favorites. Now you can just press Q to access the Align operator, which does it perfectly in one click
