Interest charge calculation prior to occupancy

Interest charged on building loan for the period between registration, but before occupancy, which is supposed to be pro-rata, is more than what the eventual interest portion will be on the final debit order. Eventual debit order will be R 35 k pm apportioned as R 23 k interest and R 12 k capital. Yet, on a 84% complete building project, I expect 84% of R 23 k interest charge but they take R 26 k. I need to understand. I need answers on this. People promise to phone back and never do. When I get put through to the 4th operator, then the call gets dropped (not on my side). Eventually when they promise to investigate and contact me during the day, they never phone.