It is really quite simple. You breed an show bird with a pet type. That then reuslts in a bird that is "half-exhibition" and "half-pet-type".
As mentioned above, show birds actually are budgies, they are just bred to enhance certain points in of their image. The whole sport of exhibiting budgerigars is when you put birds together, to get the "ideal" budgerigar. Eg. If you have a bird with a big blow )head) but a small body, and you put it to a bird with a small head and big body, you hope to get a bird witha a big head and body.
Because of this, your bird may just be a smaller show bird, or a larger sized pet bird. It's possible he may of had some show gene's mixed into him a generation or so ago, so he may display a bigger head, or a bigger bodied bird compared to a pet bird.
Hope I helped