Prusa is very slow compared to Cura as PrusaSlicer needs every option checked including Geometry handling accurate to sometimes get 7 pcs. matching Cura performance. If the result is 7 or 6 pcs. depends on where the part originally was placed on the print bed (or luck? run to run variance?). Not a reliable software for nesting.
With fast and balanced setting it only does 6 pcs.