400 ms difference between APIG latency and Lambda Latency

We have a Lambda with multiple functions fronted by an APIG. We expect that the APIG latency for a specific lambda endpoint (resource) should be very close to the p50 latency reported by the lambda for that endpoint.

However we consistently see a ~400 ms difference between the APIG latency and the various latencies for each lambda function behind the APIG.

Two questions:

1. What could be causing this and how can I test for this?

2. One hypothesis was that it might be due to SSL. I expect that SSL would only add 100 ms not 400 ms. That being said how can I confirm if the is SSL between the APIG and the Lambda, and if there is how much latency that is adding?