This has me wondering about all the posts I've read warning of grassy, or vegetal, off flavors after prolonged dryhopping.
Congratulations on discovering this myth for yourself. I've found the same thing.
It really depends on the hops used. In my experience C hops, for instance, never get grassy, but nobles on the other hand, depending on their origin I surmise, do. EKGs I have found don't, US Goldings and US Hallertaus are horrible, German Hallertaus seem to be OK. Tetts seem to be OK regardless of orign. I have found that if the flowers have a "green" aroma, they will probably be grassy as a dry hop.