Looking at the link that boaber posted, the Ark Life Managed Fund 2 dropped by 4.1% over a 3-month period to 2/1/2008. Now this 3-month period doesn't coincide with your 3-month period, but I don't think that such a discrepancy will make the difference between 4.1% and 15%.
I'd ring Ark Life and ask them to clarify exactly where the difference came from. Don't be fobbed off by vague references to market turmoil - print off boaber's linked page for ammunition.
It occurs to me that sometimes pension companies quote "nominal values" which presume you're leaving the fund with them until retirement and sometimes they quote "surrender values" which assume you're transferring the fund somewhere else. The latter can often be lower than the former if there are penalties imposed for transferring elsewhere. This may or may not be the case here.