A random thought... the appeal of PiS is basically political traditionalism; rural, ultra-Catholic, nationalist, etc. So it does best in rural areas with a settled population, like the old Polish heartlands around the Vistula, but also Galicia (maybe the xenophobic element to PiS has special appeal there as well). PO is basically just (as far as most of its voters are concerned) the anti-PiS. So it does well where a party based on traditional values and tub-thumping nationalism is likely to be less popular; in the cities (and there are more cities in the parts of Poland that used to be part of the Kaiserreich than the rest of the country) and also in areas where the population was largely dumped into after the War after being kicked out of their own homes in the east.
Might be totally wrong.
Come to mention it, the dark blue areas in the south east do correspond roughly to the formerly Austrian Galicia.
There are examples of even more ancient patterns in some French elections IIRC
