I found 'multi_accept' is 'off' as default in Nginx as follows:
Probably because with on, all the worker processes are active and try to handle all of the incoming request simultaneously. When disabled, Nginx decides which child process gets to deal with the request one by one. As Nginx is very efficient at this, this probably serves most people well. Some consider it a risk to enable it, as it may flood the worker connections with requests. Your TCP settings at OS level will likely play a part in this too.