Hi niko, so notice if you view from an angle below you can see a bit more about how this surface's end is shaped:



Notice how its side edge is fairly slanted and is not actually perpendicular to its neighboring edges?



The Blend will create a surface that wants to be perpendicular to those edges meanwhile the other piece you have there is not set up like that, that's why they are not close to one another.

What you probably would want to do is to delete these 2 surfaces here:




And create blends in those spots to replace them, that way they will be shaped similarly to the blend you already have.

That's what I did in the attached 3DM file example, which looks like this:




- Michael