There are two reasons why the historical portion of the indices may change over time.
First, a small number of late-reported transactions will enter the underlying data-set and these and other small changes to the transaction population combined with rounding occurring at different stages in the calculation may cause very small, insignificant changes to the historical values.
Second, for the segment level indices only, underlying depreciation assumptions are regularly re-calibrated within our main forecast model and changes to these assumptions can change the historic values. This regular re-calibration for the segment level indices is necessary due to the high level of volatility in the underlying price series.
These changes are very small - in the range of tenths of a percent. Consequently we don't make notations on the website nor in the file indicating that the historic value for any series has changed.
Please let us know if you have further questions.