Because it depends not only on the generated interest but in the influence of currency exchange rates. The exchange rate may differ between the time of investment and the time of return (and it is linked to the definition of Forex Influence)
Forex: Loss or profit generated by the difference on the currency’s exchange rate between the investment date and the return date.
Currency Exchange Type: Value ratio between two currencies defining how much one is worth compared to the other.
Spreads: Exchange rate difference in exchange house between the reference price (in our case coinmarketcap) and the price offered by the exchange house.