It might not be due to Bunbury track specifically, but rather where the horse can perform next (per the system). If non-metro tracks are excluded, I think it leaves York, Northam, Bunbury and Mt Barker within a 4 hour travel loop. Perhaps the limited number of available tracks might have something to say about the class of racing (???). And if as last start winner it's not favourite, a metro horse may be taking a big class drop for a run 'cause it's out of form/early in preparation (???)

How's the S/R compare with Bunbury repeat runners vs a horse running elsewhere? Maybe the course handicapper isn't the best...

What about maiden vs non-maiden last start winners? Maybe it's the done thing in the region to run your maiden through Bunbury (there are no maiden races at Ascot today, nor the previous 3 meets; and just checked the last 2 at Belmont Park: no maidens)