Linden Lab Official:I bought Linden Dollars (L$) from the LindeX and was charged more/less than I thought I would be. Why
The rates quoted on the LindeX are approximate estimates, and not a guarantee of an exact transaction amount.
When a Market order is placed, the order is filled with the lowest-priced L$ available at the time the order is placed. The market is very active, so an order seen as available might be gone by the time you place a new order.
If you'd like to purchase L$ at a specific price, consider placing a Limit order.