This is known limitation of SSAO...
(quote from manual)
"Note that for technical reasons the world grid, world axis and the lines of other non-renderable elements that are shown in the Viewport will be seen as ,geometry’ by SSAO. These should be hidden in the Viewport (Filter menu), if necessary. Furthermore, ,paradoxical’ effects may occur: SSAO darkening can occur on a Floor object at the outer edges of the Viewport."