There might be a creative way around this if it is that much of a concern.
S. 29 Work on behalf of particular unit owners or occupiers
An owners corporation for a units plan may, if authorised by an ordinary resolution, enter into and carry out an agreement with an owner or occupier of a unit for—
- (a) the maintenance of the unit; or
- (b) the provision of facilities or services for the unit (or its owner or occupier).
Perhaps, the OC could be authorised to contribute to the cost of maintaining the fence for the 5 responsible west wing units to the extent that it matches the expenditure on maintenance of the hedge, for which the OC is responsible.
As for the question of ‘How do other stratas make adjustments for expenses that benefit one group more than another?”, they often don’t and often it would be impossible. What if you had a common property BBQ, a tennis court and a children’s playground? Some might use none of these. Some might use one or two. Some might use all the facilities. The bottom line is that such common property features, or even just a nice looking hedge, ultimately benefit all, even if they don’t use them or even care about them because, if nothing else, their property value is enhanced.