No, you didn't understand it correctly. It was certainly not cost-free to invest in it earlier than 2020, despite starting point "2020". The START=2020 was by no means getting ignored. It was not possible to invest before period 2020 in either case.
But the interpolation between 2016 and 2020 affects the costs of the 2020 vintage. The beginning year of the period 2020 was 2018, and with ILED=1 the costs would be taken from 2019! And your interpolation option 5 caused the costs in 2019 to be too low.
And for the activity costs similarly: As the period 2020 is from 2018 to 2022, the ACT_COST interpolation between 2016 and 2020 affects the costs in 2018 and 2019.
But thanks anyway for the report; it was good to see what the issue was.
But the interpolation between 2016 and 2020 affects the costs of the 2020 vintage. The beginning year of the period 2020 was 2018, and with ILED=1 the costs would be taken from 2019! And your interpolation option 5 caused the costs in 2019 to be too low.
And for the activity costs similarly: As the period 2020 is from 2018 to 2022, the ACT_COST interpolation between 2016 and 2020 affects the costs in 2018 and 2019.
But thanks anyway for the report; it was good to see what the issue was.