The Green Hill graphics looking so similar to the original was intentional. If you look insanely closely, there are some differences. For instance, if you look at the background, there's a small stream on the mountains that was originally a tile error in the original. There are also some additional shadows added in, but nothing that you'd notice in a casual glance. You'd have to know every pixel of the original by heart, so needless to say, the guys at Sonic Retro have already pointed out most of the callbacks and differences in even Green Hill Zone.
The fun thing is looking at their thread on Studiopolis Zone, and seeing all of the callbacks and references there. There are a ton of really insanely subtle ones that nobody would ever know of. For instance, the Popcorn machine is based on an actual Sonic popcorn machine in Japan. There's a tiny blue logo on the part that Sonic exits through along with the popcorn, and that logo was originally from Wing Fortress in Sonic 2. The scaffolding used on the platforms came from a canceled game called "Sonic Crackers". The radio-broadcast effect from the truck is made to reference the "teleporters" in Death Egg Zone from Sonic & Knuckles. The logo on said truck came from the Game Gear. The logo on the "EGG NOG" sign in the background uses tiles from Chemical Plant Zone in Sonic 2.