Hi, this usually indicates a “bad" battery. By “bad" I don't mean, completely useless. Lithium ion batteries start to lose the maximum capacity at which it will fully charge over time.
So what happens is, the battery will be fully charged to it capacity, but not as charged as it originally was.
For instance, the Beats 2 battery has a max capacity of 56mAh, but it may only charge to 54 or 55mAh.
The best way to fix this is to replace the battery, but if you're not noticing any major decreases in battery life (battery only lasts 2 hours instead of 10), you probably don't need to replace the battery yet.
Hope this helps.