It's very straightforward...
(Current Value - Amount Invested) / Amount Invested x 100%

Amount Invested is the actual amount you spend when purchasing the units... don't include the service charge and any switching fee.

AFAIK, the chart (and its data) is accurate, there could be insignificant differences in the 2nd decimal figure due to rounding in our calculations and/or rounding when they adjusted it after distributions.

Maybe you have included the switching fee into the 'amount invested'... thus getting a lower growth rate.

edit: correction in the formula.