The player death statistics don't track all death types; they only show deaths that are attributable to a mob and these are also missing mobs like Wither Skeletons, Iron Golems, Snow Golems, Ender Dragon and Wither. (see MC-33710)

There are death message for these such as playerName fell out of the world and it counts it as a death (in general.deaths) but does not track it beyond that.

On my server, in my own players statistics.json file; the only attributable death i have is: entityKilledBy.skeleton => 1
however... it also states: general.deaths => 15

There are 14 deaths that are unattributable when they should be counted (remember; there are already death messages for a servers chat when these deaths occur).
These 14 deaths are things like falling to my death, drowning in lava, committing suicide or falling out of the world (these are all natural causes).