Today I went to a Verizon-owned store to inquire about my phone recharging issue. The clerk there gave me a much better answer than I got from the lying bastard I spoke with at the authorized dealer.
He said that because of my phone's waterproofing, the recharger has to be plugged in deeper than in most phones. If it doesn't go in far enough, the phone will "trickle charge" slowly. Then for some reason, the phone will say it is fully charged based on how long it has been charging rather than how much charge the battery has actually received. Although the problem is more frequent with car chargers, sometimes the charger that comes with the phone won't go deep enough into the phone to charge it properly.
Since sometimes my charge lasts a long time, I assume that I haven't always been pushing the plug in far enough. So now I will be sure to push the recharger plug in as far as it will go. He said that if I still have trouble, I should bring in the phone and the charger together so he can test and possibly replace them.
Hmmm, he didn't mention anything about a six-hour maximum cell-phone battery life!