As GBorreson pointed out, the Past Perfect is used to show a sequence of two past events, that is, it is 'the past in the past'. Here, however, the writer has opted to put both sentences in the Simple Past as opposed to explicitly showing their order. The nature of cause and effect here is totally clear and doesn't require the Past Perfect, but doesn't rule it out.
Although I hadn't studied for the test, I got a good grade.
is totally fine, and also quite often said by natives. However it isn't the obligatory construction. Here is another example:
Before he came, I prepared dinner.
This is semantically identical to:
Before he came, I had prepared dinner.
Although, in the second one, the sequence is far more inherent.
I hope that sheds some light on your problem!