poly resistors are created by etching a doped poly film.The etching rate depends on the geometry of the poly openings. Larger openings grant more access to the etchant and thus clear more quickly than small openings. The edges of the resistors facing outward form the sidewalls of a vast opening that etches quickly and clears early. The edges of the resistors facing inward form sidewalls of narrow slits that etch more slowly and clear later. The middle resistors lacks outward facing edges. so the final width of resistor is less than the expected. So we use Dummy resistors on two corners so that corner resistor segments have same etch rate as middle segments.
Dummy in sense of bonus cells.... that are layers that doped into the layout with NO layout functionality to the layout. Its used to improve the yield during fabrication , improves reliability of the chip.