iPhone battery symbol is red, doesn't acknowledge charging, but is apparently charged

I have an iPhone 3G,

1. When I plug in the charger, the big red battery on the locked screen is red, low and doesn't appear to be charging. When I unlock the screen, the small battery symbol on the upper right is green with a bolt.
2. When I unplug it, the battery symbol in the upper right is low and red, but the phone seems to be charged.
3. The phone syncs to my computer fine when connected.

Does anyone know what the problem is and if it can be fixed? Is it relatively benign, or perhaps an early sign that it is reaching its final stage of life?

Update

Although I am still no closer to the answer, and I had tried powering off manually, I let the battery run out (about 7 days at 1hr/day, no cell phone service) and when I charged it again, the issue resolved.

Solution 1:

Sometimes it happens when you put your iPhone for charging more time.

Please power-off and restart your iPhone. Mostly this solves problem.