Wongo's search is based on the rent that is also used for the rent subsidy calculation (huurtoeslag). The actual rent may be higher due to additional service costs, heating costs or parking lot costs that are not covered by rent subsidy.
You will usually be asked for the IB-60 form as a proof that you have low income. You can request the IB-60 form at the tax office (belastingdienst). Call +31 (0)800 0543 and they will send you the form within 5 days.