I have done several thousand BPOs and I have some thoughts on this, issue.
When ever I see sales of homes, condo, etc done by developer people, It is normally Higher than
sales done by REALTORS. You know the developer sales people work for the developer, their JOB is to get the highest price. It is not their job to give the buyer a CMA (BPO) to buyer to show them what the market value is - it's their job to move their bosses property at the best margins to the boss.
Just this morning, had 5 comps in same development, all built by same builder, all sold in past 6 months, homes very similar. 3 of the sales by Realtors came in from 415k to 456K. The two by Developer people (no Realtor buyer agent involved) came in at 575 k and 585K.
It is my policy not to use comps that do not follow the market in fair market arms length transactions, and I bet that just might have some of the same thoughts.