The case of the mysteriously shrinking Sony laptop battery

Only considered a commercial version, new and specific to the popular Dell Inspiron 1525, and wonder why the life of this new battery system was so poor compared to its very recent (and largely identical) predecessor. How can two versions of essentially the same laptop have a difference in battery life as big as three hours to two hours we found? We looked a bit further and found a potential culprit.
Both laptops $ 650 (the new 1525-139b and the largest 121B-1525) have six-cell battery, but not always the most useful number in sight. Most laptop batteries are labeled for 56Wh (or watt hours), which means they can http://indoblogger.com/laptopbatteries/ hypothetically power a one-watt load for 56 hours. The 1525-121B had a standard 56Wh battery, while the 1525-139b has only a 41Wh battery, which one would never know without the battery and read the fine print stamped on it - as the batteries (and systems) look pretty exactly the same.
Therefore, shorter life of the battery, and one more area where PC makers are concerned about their budget, apparently trying to shave a low cost of their systems. So the lesson here is that there is now a little more fine print you need to read carefully before buying a laptop.