Recently I experienced the same situation on my NTP server at Digital Ocean. The indicators showed a high amount of incoming network traffic but a considerably smaller amount of outbound traffic. In my case, the problem was being caused due to the response time of NTP requests, because of a virtual firewall that filtered traffic from my virtual machine. For every 10 requests, about 2 were not answered by the server.
After removing the virtual firewall, the inbound and outbound rates of network traffic became more consistent.
Another likely cause for high inbound traffic and low outbound traffic on an NTP server is the high CPU response time due to host processor exhaustion/congestion (phenomenon known as CPU Steal). If you are running your server on a machine that uses virtual processor cores, this is likely to be the cause.
Even if you are running the server directly on a dedicated machine, it is still possible that the NTP service is not being able to fulfill all requests due to multiple factors, including the UDP buffer of the Linux Kernel.
I recommend doing a stress test on your NTP server using the “NTPTool” software, it simulates multiple requests in a course of time to identify whether your server is being able to respond to all requests.