VirginiaBob,
Don't feel bad, this question gets asked about every 4 months. It is a good chunk of why this thread is 4 zillion posts long.
The I-fund is comprised of shares purchased from the MSCI EAFE. The I-fund and the EAFE are exactly the same thing except a little TSP magic which converts one into the other. The price of both are directly calculated based on the value of component stocks.
The EFA is a publicly traded fund which holdings are designed to mirror the EAFE, but the price is subject to buying a selling pressure just like any stock and therefore, speculation causes it to change. Please note that this speculation is the same speculation that is driving the rest of the US markets and currency markets and is usually loosely proportional (but not related too) an FV when it does occurr.
Going back to the EAFE and the I-fund, this is where the FV comes from. Around 3:00 (EST) MSCI Barra posts a price of the EAFE based upon the closing prices of the overseas stock markets, which all close a 12:00 pm EST. MSCI Barra's price is based upon the world's stock market at 12:00 pm EST. Sometime after the US markets close, probably around 6:00 pm EST, TSP purchases (or sells) EAFE shares to support the holdings of the I-fund. The price paid by TSP at this time can be slightly different then the price posted by MSCI at 3:00 because of large fluctuations in market momentum and currencies that may have occurred between 12:00 pm EST and the close of US markets. This difference is what causes the FV and why only actions that occurr after 12:00 PM EST can cause an FV.