You've set arc=15mm. The value of toptitle should at least be half of it, thus I've changed it to toptitle=10mm.
If you set line width=0mm in the titlerule style, though I don't know the reason, there ...
It has nothing to do with the size of error bar and it does not correct when you reduce the 14.52 to a lower value. It is due to the enlargelimits. I removed the messy figure in the minipage in the ...
Hmmm... I do not understand the complexity (or maybe I am missing the scope). Why you don't use the \foreach statement? This works:
You can use a x filter to multiply x by 13 like this
I wouldn't use positioning for this. Place the A node as you did, then use polar coordinates to place the other nodes. You can set angles and distances however you like for each node.
Since it seems ...
Although @Jinwen is the correct answer to the question, one suggestion to simplify the thing: why do not just use a white-filled detached title for the job, so you do not need to calculate anything?
With small changes in definitions of the nodes styles and node distance your image with use of the \small font size can nicely fine on slide:
Now considering used beamer themes. Unfortunately I ...