Sending a post like:
POST / HTTP/1.1
directly to the web server immediately sends back:
HTTP/1.1 100 Continue
And then when the post data is sent, a status 200 response comes back.
But when going through ATS the "HTTP/1.1 100 Continue" is not sent immediately, and instead is sent after the POST data has been received. This is legal, but it makes clients that are hoping for a 100 continue to wait a little while hoping to get that, ATS should forward that response through immediately.
Note: I see curl using "Expect: 100-continue" with > 1024 bytes of post data, but web searching indicates that some Microsoft products also use it.