The nominal voltage of a battery isn't the maximum voltage, 3.7V lithium cells are typically charged to around 4.2V.
A 4S battery (or equivalent) will have a nominal voltage of 14.8V and a maximum voltage of 16.8V, which leaves 2.2V extra, probably to account for losses.
Why 19V ended up being the typical voltage rather than rounding it up to something like 20V or 24V is beyond me (probably a matter of compliance, try checking IEC 60950-1 or related standards).