I think it means that there are a lot of POMO and PIMO JWs in Australia and the rate of growth of this demographic is greater than the rate of growth of POMI and PIMI demographic. Because Watchtower is looking at growth rate only in terms of actual new recruits as a percentage of baptized members, there could indeed be an increase in this figure. But the national census is looking at all who are willing to identify as JW, which would include MI X-JW, MI inactive JWs and even unbaptized publishers and MI bible students. So there must be a lot more MI X-JWs and MI inactive JWs who are going MO.
The national census reflects the growth rate of the number of mental JWs. The Watchtower figures represent the growth rate of the number of physical JWs. The growth rate of mental JWs is reducing but the growth rate of the number of physical JWs is increasing.