As we prepared the launch of 5.0 and have been showing BPT to a number of customers and analysts we often got to meet very experienced process modelers. Most of them had significant experience in BPMN and often asked, not only why we had a different notation, but also how BPMN maps to OutSystems' notation.
Regarding the first question our goal was to create a truly simple language that anyone could tackle. This implied choosing only the most common elements and make them visually identifiable at the glimpse of an eye.
To address the second question we've recently created a short two page document that maps all core BPMN 1.2 elements into their equivalents in the Agile Platform. You can find attached to this post.