The 4430 response is not as smooth as the 4435 just past 1k. There is an abrupt dip. I bet it is the from the dip shown in the response curve you provided, which I have not seen before.
Got that response curve outta one of the old JBL tech notes on the Lansing Heritage site. Interestingly, the download was available for download from there only, and not on the JBL Pro website.