Sale & Construction Agreement Confusion

I am interested in purchasing under construction flat from the builder. My builder gave me two agreements. One Sale agreement states that the purchaser approached the builder to purchase uds interest right and title and the builder agreed to procure uds of land at a consideration amount with the right to construct a residential flat through the builder. Another is a construction agreement stating that the purchaser is interested in acquiring one of the residential flats in said project and entered into an agreement for purchasing uds share interest right and title and approached the builder to construct the said apartment. But in common sense, I am purchasing a flat from a builder, not the land. Why this type of agreement? Also for the sale deed they are saying registration will be done on the sale agreement value only. Don't you think the property valuation decreases for the purchaser? Also, the bank other than approved by the builder says the same thing and they are not accepting the construction agreement because of less property valuation. What should be done in this scenario? Please guide me. Thanks in advance.