Well, this explanation on StackOverflow is even more explaininig:
http://stackoverflow.com/questions/3545219/what-are-the-differences-between-jbpm-and-activiti-bpmn
Citation from Amir Moghimi remark:
...
jBPM is quite a famous name in the Java workflow engine domain and you may be surprised to hear that jBPM 5 is NOT the 5th version of that infamous old jBPM engine! In fact, it is a re-branding of another workflow engine developed by JBoss which was previously called Drools Flow; and as the name suggests, it was part of the Drools Rules Engine project, used as an integrated workflow engine inside Drools.
Actually, as far as I know, what has happened is that Tom Baeyens, who was the founder of jBPM, and Joram Barrez, who was a jBPM core developer, have left JBoss/Red Hat (and hence, jBPM 4) and joined Alfresco to start a new workflow engine: Activiti. So, JBoss has decided to stop further developing and maintaining jBPM 4 code-base and started investing its resources on jBPM's in-house competitor, Drools Flow. But they have re-branded it as jBPM 5 to use the existing reputation and fame that jBPM has built up. Therefore, when we are talking about jBPM 5, we are in fact talking about Drools Flow.
....
This actually explains it all.
As I evaluting jBPM5 more and more, I have only to agree.
There are more disturbing news:
Citing again from Amir Moghimi):
Considering this little history, it's no surprise to see that jBPM 5 is a completely different workflow engine compared to jBPM 4 with a totally different API and documentation, lacking almost all the maturity and community support existing on jBPM 4
Cheers
Espinosa