That is an interesting analysis. There is no shortage of Boer War material at the moment so collectors are able to pick and choose more so than is usual.
I thought some of NN's estimates were high and this may have put people off? For example, DNW sold a no clasp QSA to a Captain in the Rand Rifles last month for a hammer price of £100 whereas NN estimated their Lieutenant's no clasp QSA, both having no biographical information, at £200. That said, this medal did sell and for £150.
The QSA to McDonald may well have been a "sleeper".
Whilst not spending much time researching (especially after the horse has bolted). There was a H.C McDonald who won an MC with the 10th Northumberland Fusiliers in WW!. If indeed it is the same chap, a good price I would have thought (excluding the possibility of a reunite) - extra special price.
The Australian Market is rather fickle these days, many factors come into play with NN.