https://money.stackexchange.com/questions/12706/help-me-understand-the-oddity-of-percentage-gains-and-losses is a good discussion

If you have two percent change values for adjacent time intervals, they compound rather than adding up. So 50% loss then 50% gain is base amount times 0.5 times 1.5. Which is NOT base amount minus 50 plus 50

real example — The TRBCX fund actually experienced 43% loss in 2008 then 43% gain in 2009, resulting in a 20% loss over 2 years.