I don't understand your comment. You say they test at a controlled pressure, whereas conditions in the typical medicine cabinet vary wildly, and then say that that's why the drugs expire much earlier than they should.
Do you mean that varying conditions make medicines last longer? You can't mean that medicines would last much longer if stored in perfect conditions, because that's not expiring "earlier than they should", it's expiring exactly on time for the poor conditions they are kept in.
Most chemical processes happen faster with a higher temperature. Some drugs decay into harmful substances (e.g. Aspirin).
Other factors, such as humidity or UV, may play a part as well.
Rather than testing every permutation of storage conditions over time, manufacturers put a safety margin. Individual consumers aren't in a position to remember exactly how they treated any given container of pills. Even if the manufacturer knew how a given sequence of storage practices affected their drug, nobody would be the wiser.
Do you mean that varying conditions make medicines last longer? You can't mean that medicines would last much longer if stored in perfect conditions, because that's not expiring "earlier than they should", it's expiring exactly on time for the poor conditions they are kept in.
Can you clarify?