With that said as you read the article you'll see that there are large variations in their net worth based upon certain asset classes. On top of that I don't think they're required to give an exact amount for the asset, but instead they pick a range. This makes since because not only can people attribute a subjective range to the value of an asset, but for instance the value of stocks and bonds can fluctuate considerably as well.
http://money.cnn.com/2015/05/15/pf/obamas-money/index.html