why do some aircraft have their wings set at a anhydreal when most have a dyheadreal?

Because that's the way the engineers designed them.

Sweepback has an implied dihedral effect, and so has high wing placement.

Aircraft with high sweepback and top mounted wings may have too much dihedral effect, so geometrical anhedral is required to compensate.

dihedral increases stability in roll by raising the center of lift in relationship to the center of gravity. A low wing plane with no dihedral is naturally unstable in roll, a mid winged plane has neutral stability in roll, a high wing plane has some inherent stability in roll.

A heavy high wing transport could be so stable in roll that it would be difficult to maneuver. Andihedral reduces stability in roll.

Spelling: Dihedral and Anhedral

Anhedral is sometimes used to decrease stability and increase roll rate and maneuverability in fighter aircraft. In large transport aircraft, the wing is set high above the center of gravity and the anhedral actually has a strong stabilizing effect similar to dihedral (also known as "keel effect") and such designs also have structural and maintenance benefits.