Why are dado sides not parallel?

Don, You can make a jig for your route to cut the slotdado in the sides of the case There are three methods that you can use The first is to

Freud Dado set, 207 x 6-22 x 30 For table routers and sliding table saws slots from 6 - 22 mm

ancient disco slot A dado wood joint features a rectangular slot cut across the grain of a piece of wood, into which another piece fits snugly

sostrup slot & kloster It is an EZ Dial Slot Cutter, and unlike a tablesaw dado blade stack, this router bit does not have shims, or even need to be taken apart and