Plutonium Pu-238 has a half-life of 87.7 years, i.e., in 87.7 years, 50% of its atoms decay to uranium-234 and then to lead-206.
But the RTG has already lost half its power in ~45 years. Why?
That is because the bi-metallic thermocouples, which are quite inefficient to begin with, also degrade over time.
Every year, Pu-238 degrades by 0.7872%. The thermocouples must also degrade by a comparable amount to account for the net power loss rate. See graph below.
zleap