There are a couple of things that can affect the OFA score. From th
www.offa.org website:

Radiography of pregnant or estrus females should be avoided due to possible
increased joint laxity (subluxation) from hormonal variations. OFA
recommends radiographs be taken one month after weaning pups and one month
before or after a heat cycle. Physical inactivity because of illness,
weather, or the owner's management practices may also result in some degree
of joint laxity. The OFA recommends evaluation when the dog is in good
physical condition.

Some vets just aren't good at doing the initial X-rays. OFA will
re-evaluate a new set of X-rays if you desire.

Ruth