There are several issues here.
Firstly, the distinction between FY and CY is a legacy of British colonialism. Most countries report ther full year economic metrics for Jan -Dec and what the IMF has projected is GDP for Jan-Dec 2027. Since almost all countries release quarterly GDP data, it is easy to reconcile with the national data of even those countries that use a different FY.
Secondly, just because a currency has depreciated in the past does not mean it will depreciate at the same rate or, even, at all , in the future. PKR depreciated substantially between late 2021 and 2023 , and is now almost 10% stronger than its low from September 2023.
Thirdly, the FX rate in a forecast is not an entirely exogenous variable. It is intertwined with both the growth and inflation forecast. A weaker currency will help export growth and increase inflation, which will increase the value of local currency nominal GDP and offset a lower FX conversion value to USD. After Indian Central Bank raised interest rates and started intervening in the FX market, their currency has strengthened in the last few days. The IMF would have incorporated the more restrictive policy in downgrading its forecast for real GDP growth from the latest 7.8% figure.
Finally, it is just a forecast and can always end up being way off, but the approach used by the IMF is more analytically robust than just pulling numbers out of thin air.
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