I think it may be because it virtually is a depot!
Can't quote platform numbers but to the North end of the main trainshed, there is a carriage sidings which has a number of roads as well as a train wash over the far side, parallel to the platform (8 or or 9 or something like that?)
I think coupled to the fact, some platforms see little use on a day to day basis (has to be said, it is now far oversized for what it needs to be) it makes an ideal place.
That said, I absolutely loved the station when I visited last year (me and a friend spent a couple of days based in Perth so made a few uses of the station) and similar to what you saw, there were trains literally dotted all over the place on the first day we arrived. I took a few pictures but they aren't on this PC but I think we counted about 7 or 8 units total with a couple, at least, parked up and shut down in one of the through platforms under the main trainshed (though not actually coupled)