One issue that comes up often with development land (i.e. residential subdivisions) is that the recorded sale price is based on an option price that was negotiated in some instances years earlier. Basically, a developer will approach a farmer on Jan. 1, 2000 and get an option to buy his farm at $20,000 per acre on or before Jan. 1, 2005. The farmer agrees because the market value as of the date of the option is $10,000 per acre. Fast forward five years and the housing boom has driven land prices to $50,000 per acre and the developer exercises his option and purchases the land at $20,000 per acre.
If the deed is recorded at $20,000 per acre on Jan 1. 2005, you may inadvertantly think that the market value of the parcel was $20,000 as of the sale date. However, in this hypothetical case, it is actually $50,000 per acre.
The bottom line is that you should call the buyer and seller for each of your comparable sales to see WHEN the contract price was negotiated and determine if the recorded sale price reflected the market value as of the date of sale or if it represents an option price that was negotiated years earlier.
I'm not sure if your assignment calls for a current or prospective value opinion so your supervisor may or may not be correct. You may also be incorrect if your comparable sales reflect the conditions I noted above. In any case, let us know what you find out.