imported post
From the TSP Website:
Participants have asked why, on some days, the change in the I Fund share price reported by the TSP does not match the change reported for the Morgan Stanley EAFE (Europe, Australasia, Far East) index, which the I Fund tracks. This happens when the Board's investment manager, Barclays Global Investors (BGI) reprices its EAFE Equity Index Fund, in which the TSP invests, after the close of the foreign markets. This process, known as "fair valuation," occurs when there are large U.S. market or currency movements between the time the foreign markets close and 4:00 p.m., eastern time, when BGI's share prices are determined. Fair valuation ensures that traders cannot "market time" the I Fund by making investment decisions based on the "stale" prices, thus diluting the returns of other participants who invest in the I Fund. Because the EAFE uses the foreign market closing prices to calculate its values, its price change will differ from the TSP's on those days.