NodeBalancer number of connections

I have recently started using NodeBalancer in front of one of my Linodes. I am aware of the 10,000 connections limit, which I thought I would be nowhere near. However, since using NodeBalancer, the average number of connections is ~5,500. I think I am probably missing something obvious with my configuration, but this is broadly the set-up I have:

• ~30 requests per second (HTTPS)

• Each request takes ~50ms max.

• A single server (Linode 24576) handling all requests.

• Most requests coming from mobile apps and networks.

• Requests via the NodeBalancer have the 'Connection: Close' header as I expect.

Based on the above, I would have thought that the number of connections would be in the low 100s - but it is consistently between 4,000 and 6,800 at any one time (according to the chart on the NodeBalancer page).

I am a little lost as to how I could investigate this issue. If anybody has had a similar experience, I'd welcome any pointers or advice.