There's a couple of things that could account for this.
First of all, most funds would be priced at a fixed time daily, say, noon or close of business. The value of the fund would be calculated from the price of the component shares or other assets at that time. When cashing in, the client normally receives the price at the next valuation, not the price from the previous day's valuation, which may be higher or lower. This might have been the price you saw on the website. That said, without knowing anything about the nature of the fund, you would have been very unfortunate to suffer a 7.7% drop in a single day.
Secondly, there may be a bid-offer spread on the unit price. The offer price is the price you can buy units at and the bid price is the price you can encash units at. The offer price will be higher than the bid price. Is it possible you valued your investment at the offer price, not the bid price?
Your best bet would be to put your query to BoI in writing asking for a detailed written explanation. If you are not satisfied with the response, make a formal written complaint. If that does not resolve the matter, you can make a complaint to the Financial Services Ombudsman free of charge and their decision would be binding on the bank.