Quite possibly this is how it went down. 

4 years ago they purchased a house worth $1,000,000. Put $100,000 down and now still owe $830,000. The value of the house has since risen to $1,700,000. 

But unless they sell the home it is a paper gain only. The house value could climb to $2,200,000 or fall to $650,000.  Either way, until they sell the house it means nothing.  Except to the bank, who will continue to lend the homeowner money based on the 'equity' they have in the home.........