The IRS document: 'Instructions for form 990T' only refer to assets as treated as if sold at fair market value
If the figures are indeed current market value, not cost value, then the fall of some 12% in asset values from 2013 to 2014 (two years for which the graph says actual reported values are used) is interesting. I assume that most of WT's wealth is in property, with some other investments, but S&P indices indicate that commercial property prices were slightly up over the period, while equities showed strong gains. Why WT should see its assets fall by more than $100m is therefore mystifying but one possibility is that a major financial problem hit the WT between September 2013 and August 2014.