TBH Parkfell I don't think it is necessarily a standard deviation of the students.
A lot of it is hugely influenced by quality of instruction. The flight training industry is largely inhabited by self selected instructors. Commercial considerations currently preclude most organisations from having a comprehensive standardisation effort.
I keep encountering students who aren't stupid and are trainable but who have massive gaps in knowledge. Or worst of all, have been told that what they are learning is "90% bullcrap".
And when you get a student who has just passed an IR but doesn't have a scan that includes heading, or doesn't know which way to turn to track an RMI, or who never seems to consider wind and drift, then there is something seriously wrong with the system.