Drupal 7 GetLocations field geocoding off?
The API geocoder may be different from the Maps geocoder (see the FAQ in the API documentation). The results may even vary for users from different users from different countries. For me, Spier Wine Farm, Stellenbosch results on Google Maps in -33.9733628,18.7825561.

In general, it seems that sometimes providing less information in an address results in more exact coordinates, see this similar question on s.o.

UPDATE: Interesting fact: the same address produces even different coordinates for different browsers. Coordinates stated above came from Google Chrome. When doing the same in Mozilla Firefox, I get -33.973474,18.782544 (which is what you had in your question).