[Frogm4n]

Lithium batteries use a ramp charge, where they charge the first phase at the high power you often see a device/charger rated at, and then lower the power they draw to reduce heat build up. That's why so many things advertise a "quick charge" feature these days. A high power charger will get through the first phase more quickly, but you'll always have that slow charge "top off" that it backs down to as it fills the battery. Slower/lower power chargers generally are better for the longevity of the battery because it won't get as hot while charging.

It's akin to running the tap to fill a pot at full blast when you start and then progressively slowing it down as it gets closer to full so it doesn't run over, but managed automatically by the device.

Another thing to keep in mind is that "full" doesn't usually mean finished with a lot of devices. Quite a few of them (most, in my experience) switch to 100%/green/etc. even while they are still drawing some noticeable amount of power at the very slow part of the ramp charge. I often use an inline USB power meter when charging devices and I've seen devices still take charge power for an hour or more after turning the charged indicator on. Letting a device charge overnight gives it time to finish that last bit while cooling down from the bulk of the charge cycle and you aren't tempted to take it off the charger before it finishes, and may even help the charge controller built in to the device recalibrate where full should be if it hasn't had a truly full charge in a while.