Good Morning Dixie,
While your question is an old one, it may still be of interest to you because foreclosures, foreclosure investing, distressed real estate investing and deed-in-lieu of foreclosure are all topics that people want to know more about these days.
The MLS can only track what Realtors input.
Other databases, such as Realtytrac, foreclosureradar.com, Public Records Information Source, ILFLS, and Cherry Picker (www.cherrypickerinvestments.com) focus on publicly filed data at the Cook County Recorder.
As an example, if you miss 3 mortgage payments, your lender will likely file a lis pendens to put you on notice that they are going to exercise their legal right which may result in foreclosure if you don't cure the delinquency. BUT, even though they make a public filing, the general public and certainly those using the MLS never know about this. Hence, depending on what and how people measure data, there are differences.
I hope this helps. For investors, Cherry Picker is a great resource.