Many sellers assume that they'll recoup every dollar that they put into their properties, and that's not necessarily the case. Unfortunately, if none of the other units in your complex have granite counters, similarly upgraded HVACs, pellet stoves, or any of the other modifications that your unit has, then your neighborhood comps will negatively affect the current market value of your property. In other words, one should strive to rehab one's unit towards whatever the median is in that neighborhood to maximize the amount one will recoup.
Regardless of their methodology, they should be able to make you understand it. If you can't, find someone who does a better job of communicating with you.
good luck with your sale
Ask the agents to explain to you why the other units are worth more than yours.
Also, they may think that the other units are less than yours, but are overpriced...and should be less than the amount they are suggesting for your list price.
Keep in mind that anyone can ask any amount they want when selling a property - the key component is what did they sell for. That tells you what a buyer is willing to pay and what a seller is willing to sell for. That's the definition of fair market value.
The agents may be well aware of what the asking price is for the units in the complex but they may be coming in $10K less than that because that's what the fair market value is based on recent sales.
Hope that helps! Catherine Condon, Broker NH & MA