America did not expect Okinawa to be so difficult. You can read the contemporary opinions. It’s right there. Okinawa was a shock, and changed how the US saw the war. Again - it’s not difficult to understand.
Okinawa was difficult, US realizes that an invasion of Japan would be even more difficult. US steps up atomic bomb research.. Trinity comes a month later. Bombs developed for us on Japan and used.
Name for me a place where the Japanese didn't offer fanatical resistance. Guadalcanal? Tarawa? Saipan? Peleliu? Iwo Jima? If American war planners were "surprised" by Okinawa after those previous battles then they should have been freakin' fired - the Japanese had been using that island for artillery training for years and had just about every inch zeroed. Why would they have imagined Japan itself would be anything but a bloodbath?
Again, it's not difficult to understand - they started work in 1942 - they didn't use the bomb on Germany for the simple reason that they didn't have it ready before Germany folded.
The only aspect of the battle that could be considered 'unexpected' was the massive use of Kamikazes against the fleet, although the battle of Lingayen Gulf in the Philippines gave the allies a taste.
As for whether one thing isn't like another:
Guadalcanal - 7k casulties. 2060 sq miles 36200 defemders
Tarawa - 1.7k casualties approx 1 sq mile 2619 defenders
Saipan - 13.7k casualties 45.5 sq miles 31000 defenders
Peleliu - 9.7k casualties 5 sq miles 11000 defenders
Iwo Jima - 28k casualties 8 sq miles 22000 defenders
Okinawa - 84k casulties. 463 sq miles 120000 defenders
Oki was fought in the lower third of the island where the bulk of the Japs and their prepared defenses lay. After Iwo, the allies knew what was in store for them ashore.