This process is uniquely Singapore. Sinkies don't realize that in the rest of the world, things are done much more simply.
You want to buy a resale flat, just go and find one, negotiate with the seller on the price, than put a deposit, get your financing, and than the sales shortly after. Usually, the bank appraisal matches what the negotiated price is between the buyer and seller. Why must there be a difference between the valuation and what the agreed price is? That's where you have the whole issue with the COV. In the first place, HDB should not even be doing the valuation. They are not a neutral party to the deal. AN independent 3 rd party property appraiser should be doing the valuation instead. If the seller and the buyer agrees on a price of $600K for the flat, than that is the price that the "market" will bear for the flat. ANy HDB valuation below that or above that is automatically wrong because the HDB valuation is based on what the flat theoretically should be worth, not what its market value is. A 3rd party appraiser hired by the bank will automatically support the $600K price because that is what the balance between the buyer and seller was reached.