why tostring for number use LUA_NUMBER_FMT "%.14g", not "%.16g" ?

[pulleyzzz_gmail <pulleyzzz@...>]

for this code:

a=tostring(1/3)
print(1/3==tonumber(a))  -- false

--if use .16g 

a=string.format('%.16g',1/3)
print(1/3==tonumber(a))  -- true