Klopp's number 1 ranking is a statistical one (manager "fixed effects"). The second half of the same table of results places Klopp in sixth position in terms of points won per game. Klopp averages 1.712 points per game whereas Ottmar Hitzfeld tops the list with 2.008 points per game. The statistical measure attempts to adjust for the differences in the teams managed by the different managers. The difference between the two measures can be sizable. For example, in second place on the points per match list is Giovanni Trapattoni whereas the Italian is ranked 36th behind Klopp on the statistical measure.
Adjusting for team budgets makes little difference. Klopp continues to rank number 1 amongst the managers with more than one club - also known as mover managers.
Interestingly, the ranking for the combined group of mover and non-mover managers is relegated to Appendix 1. Here Klopp is ranked number 4 on the fixed effects measure (behind Thomas Tuchel, Pep Guardiola and Peter Pacult) and 13th in terms of points won per match.
The fact that Tuchel tops the list of all managers is interesting. For the data period covered in the study he managed one club - Mainz. He is now manager of Dortmund. Klopp had a similar career path (a comparison of the two is provided here). It made me wonder if there is something in these two clubs that is not picked up in the study. It could be that Mainz and Dortmund just happened to get two good managers.
The article is titled "The Contribution of Managers to Organizational Success: Evidence from German Soccer" and it is written by Gerd Muehlheusser, Sandra Schneemann, Dirk Sliwka, and Niklas Wallmeier (here).