This is not at all surprising, and is very expected.
To be realistic, and generally speaking, diploma holders would have limited capacity/proficiency to perform, especially when compared to higher qualified or higher experienced peers. This is true across all disciplines. Even degree holders have to learn on the job because whatever knowledge that was learnt in university is either too fundamental (to design/build/create/produce) anything fantastic, or else, just outdated.
In my post, I included persons with a diploma of photography only because they have formally studied it. They may not be superb photographers.
In fact, even degree holders may not be superb photographers themselves, and in reality, many aren't and many never will. Like everyone else, they may have a photography degree (which they can flash around and show off) but their transcripts may have a long list of grade E (the passing grade) for every subject.
Here in Melbourne, I met one RMIT photography graduate who told me that he knew and heard of graduate peers who quit and flee the photography industry. Their crime was simply because they unskillfully screwed up a wedding job badly enough and know that they will never be able to easily recover their reputation (despite holding a recognised degree).