Even the death numbers reported in 2020 and 2021 are not final. The more than 21,000 deaths Kerala has reported in the last four months have not all happened this year. Most of them pertain to last year. The 1,300 deaths Assam added on April 25 did not all happen that day, or that month, or this year. They most likely happened the previous year. Several hundreds, possibly thousands, of deaths that states adjusted in 2021 would have actually happened in 2020. The additions to the overall tally are being made on the day these deaths are being confirmed, and not the day these might have happened.
It is difficult to measure the scale of the undercount in a situation like this, particularly when the counting exercise is still on. A physical count, and verification, of the dead in a country as vast as India during such chaotic times is bound to take a little more time than running some equations in a computer model.
The WHO report does not get into calculating the scale of the undercount, for India or any other country. It has done a more straightforward exercise of calculating excess mortality. It has estimated the total number of people who likely died in India in 2020 due to all causes and, from that, has subtracted the expected number of all-cause deaths if there was no Covid. These ‘excess’ deaths are considered to be a direct or indirect result of Covid-19.