Hi Joe, I emailed back just a little while ago.
I think I found the problem. The 28″ side mount slide will fit in a 702mm deep opening, the next slide down, the default 27″ Blum undermount slide says 709mm. I think that was throwing it off. I think the software is looking at the first column to determine the longest possible slide that fits, rather than the last column, like I was thinking.t