Yes, the number of household members does affect the final settlement.
If the number of household members used in the final settlement differs from the number of household members who were registered in the rental housing during the final settlement period (previous year), the applicant can request a revision by email to hms@hms.is.
The email has to include the applicant's arguments and documents that confirm that this was the case.
This can for example be confirmation of change of legal address from Registers Iceland (Þjóðskrá).