I've done the math on the STO PAST test. If a person performs all exercises to the exact time limit then why does the package give a 3 hour maximum time limit?

Good point. I always considered the 3 hour limit as a guide for the PAST Monitor to terminate the test and say, "Hey, that's enough. Go train some more and come back another day!"

This has puzzled me as well. In the PJ PAST, even if you just barely squeezed by on every single time limit, and used every second of your break times, it would still come out to around 90 minutes, which is obviously considerably less than 3 hours. If someone is taking 3 hours to complete the PAST, they would have had to essentially double the amount of time they used to do everything; 22 minute swim followed by an hour break, 20 minute run followed by 20 minute break, etc.
I mean, the 3 hour limit isn't technically incorrect, it just seems redundant in that for the limit to be reached, you would have had to fail every single test in a colossal way.

The PAST published in ARRSR 55-11 in 1977 and 1982 required 4 count flutter kicks, 8 count body builders, 2 count chin ups, 2 count pushup, 2 count sit ups, 2000 yard swim, 3-mile run.

Calestenics were required to be completed within a 30 minute time frame. The three hour rule is perhaps something that has not changed when other standards of the PAST changed. For example the eight count body builder was off the PAST by 1996 and flutter kicks were dropped sometime after the year 2000.