Why am I seeing ELB health checks doubling up?

I have an ELB with the following configuration:

• Availability Zones: ap-southeast-1a and ap-southeast-1b
• Cross-Zone Load Balancing: Enabled
• Connection Settings: Idle Timeout: 300 seconds

Health Check Details:

• Ping Target HTTPS:443/login
• Timeout 15 seconds
• Interval 20 seconds
• Unhealthy Threshold 5
• Healthy Threshold 3

Listener Details:

TCP on port 443, forwarding to instance port 443, where nginx is listening and doing ssl termination.

I continually see double health check calls in the nginx logs. The come at the same moment, at least the same second anyway.

Why?