interesting statistic... 
To be more precise

At my school there are classes for those transition in similar instruments
That simply mean that if have already grade 5 or 9 in piano by also beginning an organ class you don't start from the scratch
Students with a diploma in piano moving from piano to organ begins at the 4 grade level while people moving from organ to piano begin at the 8 grade
That means that for someone who already mastered the piano it will took 6 grades or years to master the piano, while from someone who mastered the organ it will take 2 to 3 grades or years to master the piano
This is probably due to the fact that moving from piano to organ you have a lot of extra stuff to learn such as more keyboards, not sitting on your feet, reading the baritone clef, using the pedals
While switching from organ to piano you basically have to just get used to the feel of the key and the pedaling, all the rest will be just practicing repertoire because you aquired all the technique needed from piano studying organ (as a matter of fact the most complex pieces for organ are way harder and demanding more technique than any virtuoso piece for piano)
There's a lot of new technique to learn when you switch from piano to organ while there's almost no new technique to learn when you switch from organ to piano
Daniel