It is absolutely true that certain cultures tend to give higher scores than other ones, but it is very difficult to determine the regional impact.
It seems like the managers in this study are doing a very good job indeed if they only get 9’s and 10’s. NPS is not the ideal measure to measure manager’s performance, as it was developed as a customer satisfaction index. I would suggest to use a number of different statements about the managers conduct/performance using a 5-point Likert scale (‘totally disagree’ to ‘totally agree’). Or even better a 360° exercise. In our survey tool you can find a template for a 360° evaluation questionnaire. You can register for a free trial to check this out.