What does cell phone wall adapter specs mean?
It is to help allocating how much you can connect to a power strip or after a fuse, so that fuse does not trip when plugging devices in or flipping a switch on, as the initial surge current is higher than operating current. Chargers generally are quite efficient so when 10 Watts goes to phone, the charger could take 12W maybe in, if it takes 20W then it is a pretty stupid charger. Some parts of the world like California have pretty strict rules how much devices are allowed to waste energy.