Could be related to Bitdefender Free's lack of some protection features that are available in the paid like AMSI, memory scanning, command line scanner and some more.
For example, the PowerShell based ransomware that I talked to you about a few days ago which was instantly detected by BD paid version after executing thanks to AMSI while BD Free didn't react in my first attempt and on my second test it was detected by the behavior blocker but not before desktop files were encrypted.
Kaspersky Free and Avast Free are more complete free products.
I also checked those 5 missed by BD and two pair of them were the same malware variant basically. A more diverse set of samples would've been better, but probably hard to find.
