I presume the initial selection is smarter than drawing names out of a hat. The assessment invitation talks about matching a particular profile they are seeking, and I'd guess it is more complex than: a degree, some flying, mid twenties. Maybe they look for evidence of particular characteristics - determination, leadership, for example? So it may be to do with the types of work you've done - or more subtly, the keywords you used to describe your responsibilities in those posts. We can also deduce, because they want to see the certificates as proof, that they give a lot of weight to GCSEs/A-levels.