Shares in the FTSE 100 giant closed down five per cent yesterday on news it suffered a 0.6 per cent fall in funds under management in the quarter, to £322.5bn from £324.5bn at March 31, following hefty withdrawals by clients.
These included £4bn of funds invested in areas like Asia Pacific and global equities withdrawn by one client. Clients took £3.3bn net out of the Scottish Widows Investment Partnership business Aberdeen bought for £550m in March.
Regarding the institution that withdrew £4bn, Mr Gilbert said: "It's fair to say it was a 2013 performance issue."
The company did not name the institution, which is still a client.
Mr Gilbert added: "We had a bad 2013 so you have to expect that we do lose business, but we had so many good years before that it was not surprising."
Aberdeen suffered in 2013 as investors cooled on emerging markets, in which it has invested heavily. Investors were concerned about the US Federal Reserve unwinding measures that supported the global economy.
However, Mr Gilbert said Aberdeen's performance has rebounded strongly in 2014.
Mr Gilbert said the funds withdrawn from SWIP included some losses Aberdeen had expected. Some withdrawals related to unspecified seasonal changes made by corporate clients.
Asked whether the withdrawals raised questions about the wisdom of buying SWIP from Lloyds Banking Group, he said: "I think it's a tremendous strategic fit between Aberdeen and SWIP and I know Lloyds shares that view." Lloyds took shares in Aberdeen as payment for SWIP.
Mr Gilbert said Aberdeen is pleased with the progress of the integration of SWIP.
The company completed the purchase of the bulk of SWIP in March. The acquisition boosted Aberdeen's assets under management by around £140bn.
Mr Gilbert said there could be job cuts in areas where there is duplication but the acquisition of SWIP had not been driven by the expectation of making massive cost savings.
On the prospects for the enlarged firm, Mr Gilbert said: "The business is still in really good shape."
Aberdeen said it remains cautious on market sentiment in the short term but is confident it can grow revenue and profit without further acquisitions.
The company noted the majority of the outflows for the quarter to June had been from lower margin products. The effect on annual fee income is relatively small.
Some £5.5bn net was withdrawn from the core Aberdeen business including £4.2bn equities, £0.6bn fixed income and £0.7bn from other investment products.
Aberdeen said it had won over £2bn of mandates that were not funded at 30 June, including Asia Pacific and global equities, emerging market debt and property.
Market movements added £6bn to group funds under management in the quarter to June, net of a £5.3bn negative foreign exchange effect.
The purchase of the SWIP infrastructure business in May added £800m.
Clients withdrew £8.8bn funds in total, net, in the six months to March 31.
Justin Bates, analyst at Liberum Capital, told clients: "Aberdeen has proven itself to be one of the best performing (operationally) asset managers in the sector in the last five years. We see further scope for upside following the integration of the SWIP acquisition."
Aberdeen Asset Management shares closed down 24.4p at 435p.