Why do HKS coilovers come lower at the rear?
 

My HKS SPs came with .75" drop in the front and .83" in the rear, and I was also watching a Sarahntuned video and her Forester's HKS GTs came with 1.2" drop front 1.8" drop rear. Is this common on all coilovers or just HKS, and is there a valid reason for it?

It is common on some coil overs, I feel like it is to lower the overall rake of the car. Stock the rear is about half an inch higher than the front so to make the car sit more "flat" the rear needs to be lowered more than the front. .08 of an inch isn't much to worry about though.