Good question. I would hazard a guess that they revised the test every year, but didn't want to put the more recent copies of it in the library as they'd be similar, and could poison the results. And therefore they held back old versions as "secret" for, say, 30 years. (Thus your later case, that the test was placed in the library 30 years after it was used.)

Or perhaps this was lost under someone's desk for 30 years in the admissions department, and when they moved into new offices they said "we should put this in the library for posterity's sake."