It took me a few minutes to work out what you were suggesting but I got it. Unfortunately it didn't resolve the problem, but it sort of seems like it should have.
Ok, I was wondering if it was our template brought forward from 19. So I loaded up the 20 template and W/E and got the same issue.
If you want to replicate the issue:
1. Fire up AC20
2. Create a New Project, use Archicad 20 residential.tpl, W/E Standard Profile 20
3. Click on Slab tool, it will be on a 6" thick slab with Offset to Home Story as 0", and on Polygonal method. Go ahead and draw a slab.
4. Now change the Offset to Home Story from 0" to 5'. Click once to start drawing a slab and note that Distance is now 5'. Draw a leg of the slab as 10', and then another random point, and close the slab as a triangle. Measure the 10' side, it won't be 10'.
5. Now click on Geometry Method: Rectangular and draw a 10' x 2' slab. It'll be 5' off the Home Story and correctly sized.
AC19-23 | Win10 Pro | i7-8700K | Quadro P600 | 500gb 960 EVO M2 | Dual 28" 4k Samsung U28E590