First off, I’ll move this question to to proper topic area…I am assuming you are building EHBP-01…
Yes, there should be a gap at the top. It won’t be seen. You can tightened it up if you like, but I wouldn’t worry about it. The gaps can be covered with trim.
Look at page 12 of the plans document…you can also see the small 1/2″ gap at the top of part 17. That small gap is hidden by the overhang of the bar rail (part 18) or, like I said above, it can be hidden with an optional top trim strip.
It’s hard to say exactly why it’s 1/2 shorter than needed, probably to allow for easy installation and rounded down to the nearest acceptable whole number. That plan was designed in 1996 and it (the gap) hasn’t been an issue for anyone since.
BTW, you can always make your finish panel (16 & 17) 1/2″ higher and it will cover your OSB (14 & 15)gap.