BB writes percipiently and even presciently: is it simply a case of people deciding that part of an existing structure should be considered as a separate body part in its own right
short answer yes....
God didnt design a knee and think- Oh I need two menisci - and a few popliteal ligaments No I like them - I will put in two....
They develop and are as a result of evolutionary pressure reasonably fit for purpose.
Given that - you have in the nineteenth century - is this one muscle (in two parts ) or two muscles? The main arbiter was Henry Gray (1858) and the main one before that was Quain (1838 ) . How different was one from the other ? well I have never met anyone who has read Quain, quite honestly.
It is surprising - in the nineteenth century you would have thought they got it straight, ssince they had nothing really else to do on those long winter evenings but cut up bodies......