Why is Zendesk giving 429 rate limit error even when we honor the rate limit headers ?

We are trying some performance test on one of our application using Zendesk API. But we see multiple 429 errors even when we are running our test with 5 request per second which far below the allowed limit.

Please look at the following response header screenshot which clearly tells that we are allowed to make 700 request per minute: 

Now look at the following screenshot which shows multiple 429 errors even with 5 request per second:

Please note we tried this test on create ticket API
Let us know your feedback on this.