 According to the CFA Institute's Global Investment Performance Standards (GIPS),

This is because an annualised rate of return over a period of less than one year is statistically unlikely to be indicative of the annualised rate of return over the long run, where there is risk involved.Annualizing a return over a period of less than one year might be interpreted as suggesting that the rest of the year is most likely to have the same rate of return, effectively projecting that rate of return over the whole year.Note that this does not apply to interest rates or yields where there is no significant risk involved.