Newbie: Question about pre-foreclosures

[Quentin Thiery]

Hi all! I am a young aspiring real estate investor who is working on educating themselves about lots of different aspects of REI. I am currently reading this one book "The Real Estate Wholesaling Bible" by Than Merrill. I am interested in getting into real estate through wholesaling. I know it requires a lot of effort, but I am willing to put it in the work. I have a question about the following line in the book:

"Also realize that when you start prospecting for pre-foreclosure properties the majority of homeowners you speak to will owe more than the property is actually worth"... and continues to say a short sale will need to be negotiated 

My question: How is that possible to owe more than the property is worth? Say you have a 100K property and put 10K down and a 90K loan and monthly payments are 1K (simple numbers) and you miss 3 months of mortgage payments. How exactly does this homeowner owe more than what the property is worth considering the property is worth 100K and the loan is 90. This may be a very obvious answer and I apologize if it is as I am really new to this and I'm trying to learn more and more everyday. I hope I can get some insight from some of the more experienced investors our there and if there are any books you recommend - I am open to suggestions!

Thank you so much and I appreciate your time!

Quentin