Annualised investment return... am I missing something ?

Here's a typical table of annualised investment returns. The month and ytd figures are easily understood.... the ytd being the sum of the monthly figures. What I can't get my head around is the big swings in the other figures. I assume that the %1YR is a rolling figure so the change from 9.58% in April to 18.60 % in May says to me that there must have been a big change in May/June last year . Why does the Feb to March figure in %1Yr drop from 8.32 to 7.48 yet the %3Yr figure for the same period go up from 14.47 to 15.45 ? Even more puzzling is the %5Yr column.... in Jan the annualised return is 8.32 yet by September this leaps to 15.10. Is this saying that an investment held in Jan for the previous 5 years would have returned 8.32 percent every year ? If so , why would an investment held in September for the previous 5 years have returned 15.10 percent every year ? I know we've been through some stockmarket trauma but the numbers seem to fluctuate wildly. Any one able to explain ? Investment Results Month % YTD %1YR %3 Yr %5Yr January 0.14 0.14 9.50 14.99 8.32 February 0.12 0.26 8.32 14.47 8.43 March 2.39 2.65 7.48 15.45 8.26 April 2.20 4.91 9.58 15.86 9.06 May 2.77 7.82 18.60 17.64 9.70 June -0.94 6.80 15.85 16.49 11.0 July -2.24 4.41 12.47 16.32 12.38 August -0.02 4.39 11.60 15.62 12.16 September 3.12 7.65 12.98 15.93 15.10