Xara :: Extract Logic Behind Handling Of Multi-stage Blends?

I've been trying to extract the logic behind Xara's handling of multi-stage blends, but am having trouble.

Here's an example. I have these objects:

and create a blend between the left two

now, to continue with a multi-stage blend, I connect the bottom control point of the second object with the bottom control point of the third

All well and good. However, if I had tried to connect the top point of the second object with the top point of the third, like this (screenshot taken just before I release the mouse button)

Xara decides to blend from the first object to the third.

Eventually, by trial and error, I can get my multi-stage blends working, but it's inscrutable why it works logically sometimes, and why not.