decomposePar -allRegions produces different results than decomposePar -region XXX

Steps to reproduce

heatTransfer/chtMultiRegionFoam/windshieldDefrost

Set system/decomposeParDict to scotch

decomposePar -allRegions

vs

decomposePar -region exterior
decomposePar -region ice
decomposePar -region cabin

• eg simple method produces exactly the same decompositions
• we compile scotch with SCOTCH_DETERMINISTIC

bug or feature of scotch?