Okay, here's some useful material you can read to better understand the process of offer, counter-offer, etc...
You'd have to read the documents you possess to know. There could be many reasons.
First off, you make an offer.
The seller accepts or rejects.
As with most things in life, it's all about the money.
There are ways to avoid this.
Last, but most importantly, don't sign any contract, especially a real estate contract until you've run it by YOUR attorney.
That's the best $500 or $1,000 you can spend when making a six or seven figure purchase.
One more thing, the seller's agent is there to make money, YOUR MONEY.
Proceed with great caution.
Here you go, read all about it:
http://www.realtor.com/home-finance...dling-real-estate-counter-offers-sellers.aspx
http://realestate.msn.com/article.aspx?cp-documentid=23599546
http://homebuying.about.com/od/offersnegotiations/qt/CounterOffers.htm
.........