My curiosity is piqued ....
I would like to know her lines PM me (dont put on public forum) ... I do know of a litter of 9 which all turned out to be on the small side... just never reached the height, they are now 3yrs going on 4 years... AND know the dad has produced some 'shorter' dogs, although have to say, very proportioned and 3 of the litter did well in the showring qualifying for crufts etc, until they reached Junior...
Including my Summer

:D my mighty midget ;)
She isnt that short on leg, but against some of the taller dogs she looks positively 'dwarfed', but in real terms against the standard is just a half inch below requirement....
Like your girl, her withers are just below the knee....
If she is in proportion, ie: length of her back is not much longer that her legs, then she is simply small.
Bear in mind.. many many Labs out there are very much taller than the breed standard,
look at your girls attributes against the breed standard, not other Labs :D