Well, that's a tricky one. There is no mistake there, well, not on my part anyway. This was one of the hardest things to spot in this set for me, it took me a long time to crack down. On that day Lendl received 10 extra points, but not for Brussels, it was for US Open 1982 (it seems the ATP missed 10 bonus points for him or gave him an extra for it - I really can't tell if it's an error or intention). I initially thought the points were for Brussels and I gave it 118, but in 1984 he drops 107. More than that, I spotted he drops 225 for Canada and that one bonus point was assigned in September. I spent ages trying to realize what I was missing, the ATP has done it before to miss bonus points, but this was especially hard because the source was not as obvious. So, I somehow got to September 1983 and noticed 10 extra points dropping. I initially thought I had missed the San Francisco stars but that was affecting multiple players, he was the only one affected. After some deeper crackdowns I pinpointed US Open 1982 dropping with 10 more points than entering, connected the dots to the 11 extra points I was getting at Brussels, and left it all there just like the ATP did it. It seems it was not an error to have the 10 points, but it may be an error that he did not get the 10 points from the start (US Open 1982). I wanted go back and update those rankings as well, and there would be a slight difference at the 1982 year end rankings, but I didn't yet come to the conclusion if it's worth it. Lendl ending with 1390, instead of 1380 gives him 0.833 which is not gonna place him in front. He would only get one extra week at #2 instead of #3 on October 4, 1982. Shouldn't be too hard a task, and it would clear that slight confusion there.I spotted another possible mistake with Lendl. It is not clear how many points he got in March 14 1983 for the Brussels event. In your current Excel sheet he gained 107 points, yet effectively it is 117 in the next column. Maybe he got 12 bonus points instead of 2. And the same uncertainty arises when these points are deducted in the spring of 1984.
When I last checked your Excel sheet in March, you awarded him 13 bonus points and thus 118 total points for his Brussels performance.