Downloaded package (xxx.pkg) can't be opened because it is from an unidentified developer

I'm the developer of an application which clients can download from my website (lightroomstatistics dot com). The first time the clients / or I, on a different computer, start the downloaded installer I get the waing:

"LR Analyzer.pkg" can't be opened because it is from an unidentified developer."

However when ruing the package a second time, without changing anything, the installer runs fine. Also I can view the certificate info, see image 1. My test machine is a MacBook Air and runs on OS-X El Capitan.

Certificate info Package installer

Question: why do I get this message since I'm signed my application and package.

Further I enrolled in the Apple Developer program and signed both my app and the package. BTW, the app itself never gives a waing, only the package I download.

For packaging I use Packages from WhiteBox. After packaging I run this command to sign my package:

productsign --sign "Developer ID Installer: Dirk Gerard Peter Holtman" "./build/LR Analyzer.pkg" "LR Analyzer.pkg"
spctl --assess --verbose=4 --type install "LR Analyzer.pkg"

The result:Console feedback signing