Don't get too hung up on it -- there is really not much meaningful difference whether you are classified as 7a or 7b (and honestly, your property may well behave as a different zone than *that*, entirely or in part, because of microclimates).
And yes, the Sunset western zone scheme numbers things entirely differently. Just make sure you are comparing apples to apples when you read plant descriptions, and you will be fine -- don't try to mentally convert between the two because you CAN'T.