You asked a very good question. A Realtor has more leeway in his or her choices of comps for a market analysis or broker price opinion than an appriaser does.
The new condos could have closed at a higher value than todays market value because the buyer had a non-refundable deposit. For example if the buyer had a sunk cost (non -refundable deposit of $50,000; the contract and closing price was $300,000. --- It would made economic sense for the buyer to accept delivery of the condo even if the market value had dropped all the way down to $260,000.
So while that transaction made sense for that particular buyer, the sale recorded at $300,000 does not justify a valuation of another unit at the $300,000 price.
A proper valuation estimate would explain in the narrative and in the valuation columns the higher price due to the buyers sunk costs