Deviations from the interest parity are possible, one of the factors can be, if the earnings on account of transactions in foreign currency are treated as capital gains, in home country or in foreign country, and hence are taxed at rates different from the rates applicable to interest income, though higher or lower. The forward exchange rate is determined by a parity relationship among the spot exchange rate and differences in interest rates between two countries, which reflects an economic equilibrium in the foreign exchange market under which arbitrage opportunities are eliminated. When in equilibrium, and when interest rates vary across two countries, the parity condition implies that the forward rate includes a premium or discount reflecting the interest rate differential. So if the Forward Rate and Spot Rate are in the the forex market convention (and not textbook convention), and the pair is USD/CAD, USD interest rate is 0.25% and CAD interest rate is 0.75%, you can infer that Forward Rate for USD/CAD should be higher than Spot Rate because USD has lower interest rate.