Alijaz is a 10-dancer, so expert in both ballroom and latin. Nat is a ballroom expert, Janette is latin -- and limited latin at that, I believe. So for dances that are more ballroom than latin, it makes more sense for Alijaz to dance with Nat than with Janette, especially as the height difference between Alijaz and Janette is very likely a problem when in dance hold.
Originally Posted by James_Laverty:
“Indeed, and even when she isn't the centrepiece, she's still near the front.”
That's probaby because Janette is one of the shortest, if not the shortest, female pro. Height is why Nat is always at the back.