It does not seem to be an across the board thing, just Suncorp and AAMI (apparently Suncorp bought AAMI recently, so basically the same company). And RACV had lower premiums for including fire & theft than without.

The comprehensive insurance gives you a few more options with excess and amount insured and I thought it might be the difference, but even making sure the excess was the same it was cheaper to have comprehensive. I won't complain - but I will budget for an increase next year in case it is a one-off glitch that won't be repeated!