The average risk weight on mortgages across banks "will fluctuate" while the industry and also the regulator tinker, ARPA has cautioned.Speaking at a Finsia forum in Melbourne on Friday, APRA chair Wayne Byres provided perspective on the uneven growth in risk-weighted home loans at major banks over the September 2016 quarter.Banking Day reported on this topic last week. For Westpac, the rise in mortgage RWAs was 46 per cent. For ANZ the rise was 42 per cent while at Commonwealth Bank risk weighted home loans lifted by 32 per cent.In July 2015 APRA announced changes to the treatment of residential mortgages for banks able to use internal models for capital adequacy purposes (in other words, the four major banks and Macquarie).At the time APRA adjusted the risk weight calculation used and said it "was intended to increase the average risk weight on Australian residential mortgage exposures, measured across all [five banks], from approximately 16 per cent to an average of at least 25 per cent."Byres told the Finsia forum: "In July 2015, we announced higher mortgage risk weights for banks using internal model-based approach to capital. This was an interim step, but a step we were comfortable we wouldn't need or want to materially unwind, regardless of the outcomes in Basel."All other things being equal, we expected to raise the average mortgage risk weights for banks using internal models from around 16 per cent to at least 25 per cent. "Unfortunately, in the world of internal models, all other things are rarely equal. Banks constantly refine their models, often at their own initiative but also sometimes at the request of APRA. "We noted earlier this year that the impact of a range of modelling changes in the pipeline, when combined with the adjustment proposed in July 2015, would have produced an average risk weight well in excess of our interim objective of 25 per cent. So we've had to slightly recalibrate the adjustment, with a view to ensuring the outcomes were broadly consistent with the target we announced."I mention this because, for those who follow these numbers closely, there will be some noise in the system over the next few quarters. "As various modelling changes come on stream, the average risk weight across all IRB banks will fluctuate somewhat, and will impact different banks at different times. But these differences will narrow over time."