I'd like to preface this with the statement that I am entirely unfamiliar with any South African standards, so take my educated guess for what it is.
I was able to find what I think is the correct version of SANS 2001-CS1:2005 online. I'm assuming your confusion stems from the seemingly contradictory nature of Section 220.127.116.11 with Section 18.104.22.168, which are copied below:
22.214.171.124 Bolt/nut Combinations
The combination of bolts and nuts shall not be less than the following:
a) class 4.8 bolts or screws with class 4 nuts in accordance with SANS 1700-5-2; or
b) class 8.8 bolts with class 8 nuts in accordance with SANS 1700-5-2.
Any bolt assembly which seizes when being tightened shall be replaced.
126.96.36.199 Galvanized nuts
Nuts that are to be hot-dip galvanized shall be of a higher class than the associated bolt or screw. Nuts shall be checked after being galvanized for free running on the bolt and shall be re-tapped to the permissible deviations if necessary to ensure a satisfactory tightening performance.
My guess is one of two things:
either the hardness of the nut is expected to decrease after the hot-dip galvanization process (which dips the nut in molten zinc, thereby tempering it a little), thus "downgrading" it, or
galvanized nuts are sold to a different class system, so you have to make sure your nut class exceeds the bolt class (I don't know how things work in South Africa, so this is a wild guess).
Section 188.8.131.52 shows that a "matching" nut is used with the corresponding bolt. You want a nut that's stronger than the bolt, such that if the bolt fails (fractures), it's very apparent that the failure has occurred. If a thread fails, it's much less noticeable, and hence more dangerous since that bolt assembly cannot be counted on. See this PDF for more information on the topic.