You are mixing a bit of apple and oranges here.
Temperature in ISA is defined as 15°C at sea level with a lapse rate of 1,98°C/1000ft until 36000ft, when it stops at 56,5°C and it is then the same for quite a while.
So at FL380, ISA temperature is always -56,5°C. In your case, ISA deviation at FL380 is -4°C, which matches the -61°C (if you round the numbers) and is correct. Life - fortunately - is never standard, so forecast for the actual atmospheric conditions during the flight are that tropopause is at 43400ft (as opossed to ISA's 36000ft), so if you climb higher than FL380 in that atmosphere, the temperature should continue to drop.