Why are there so many definitions of Project Portfolio Management?

Several years ago when Project Portfolio Management was starting to get more attention and recognition, I worked for a Project Management software company. Those of us in the industry recognized the need to offer a Portfolio Management solution. So we claimed we had that because we gave organizations a view of their projects across the organization. This was a great capability but missed the major value of Portfolio Management. Portfolio Management (PfM) should bring maximum ROI from the organization's investments. Project Portfolio Management is focused on aligning projects to corporate strategic to achieve the business goals. So Project Portfolio Management does look across all the projects of an organization (so does the PMO) but the definition doesn't stop there, the key is the value that the PPM process brings to the business.