I wasn't on the flight deck, nor do I have inside information, so this answer is an educated guess.

I assume this is flight-training for new Beluga crews and/or license re-validation which is included in normal line operations for the sake of efficiency. 

There are no spare Belugas, so if new pilots are trained or some kind of license re-validation is needed (e.g. after prolonged leave due 
 to illness / maternity/paternity-leave), training needs to happen on those aircraft that are used for day-to-day transport operations. 

To minimise the impact on the transport schedule, it makes sense include the training in scheduled transport operations, for example by including a few go-arounds at the destination airport. This way, training has the least impact on schedule and it keeps the cost of training low.