Although I was told to charge my Android device before first use, it was already charged. Have I been given a phone rejected by someone else?
It is not uncommon that rejected/refurbished phones find their way back to the market as "new" ones.

But in my (and at least Al Everett's) experience, batteries are almost always shipped with some charge. I wouldn't worry, as long as there are no other indicators that your device was pre-owned.