)griffin went to new orleans, louisiana.

on his first day there, he walked around the city as a white man, dining at fancy restaurants, going wherever he wanted to go and doing whatever he wanted to do.

the next day, he visited a doctor to have his skin darkened artificially so that he could pass as a black man.

the doctor gave him medication that would stimulate the production of melanin and had griffin spend hours under ultraviolet lights.

griffin’s skin turned dark brown. though he had chosen to do so and knew that the tan was only temporary, griffin was shocked by his changed appearance.

going out into the city as a dark-skinned man, griffin was surprised at how limited his choices had become.

밑줄친 부분에 대한 질문인데요, 저기서 had become 그러니까.. 대과거가 쓰인 이유가 뭔가요?
그리핀은 놀랐다. 그의 선택이 얼마나 제한됬는지에 대해서. 놀란것과 제한과의 시제차이가 있다고 생각되지 않는데 왜 대과거를 쓴건가요?