It's just the usual confusion caused by different stuff in different materials. The CAAP says:
6.1.1 ... When arriving at an aerodrome to land, the pilot will normally join the circuit on upwind, crosswind (midfield), or at or before mid-downwind.
...
6.8.2 Joining on the base leg is not a standard procedure. CASA recommends that pilots join the circuit on either the crosswind (midfield) or downwind leg.
...
6.6.4 Aircraft joining on the active side at circuit altitude should enter midfield at approximately 45 degrees to the downwind leg, giving way to, or following, aircraft already established in the circuit.
The only textual reference in the CAAP to something happening between midfield and the upwind end is:
6.6.3 Aircraft descending on the non-active side of the circuit should be established at circuit altitude by the time they cross the runway centreline on crosswind, somewhere between midfield and the departure end of the runway.
The CASA Education Officers say it should be midfield, not somewhere between midfield and the upwind end of the runway.
Of course, the AIP contains the authoritative rule, and the content of the CAAP and the CASA Education Officers' opinions are interesting but not authoritative. However, some people read CAAPs and listen to CASA Education Officers.
The tendency to join over the middle of the runway has come from those who like to hear themselves say the extra words, "joining mid-field crosswind".
I think you've overstated that.
I'd suggest it's because everyone with a little bit of experience knows that, as a consequence of this confusion or deliberate decision, some people join at midfield, some people join at the upwind end, some people join in between, and it's a good idea to give other people a hint as to which one of those you've chosen.