The reasoning seems to be that Tomiyasu makes sense at RB despite being a bit unorthodox because we'll deploy Tierney on the left.
It's Tetris selection but if Tierney strikes good form and White grows a bit further in confidence and consistency, the balance of a Tierney - Gabriel - White - Tomiyasu back four seems better than we've had for some time.
Tomi with his blend of defensive attributes, mobility and ball-playing attributes will be useful in other formations too, as will White (maybe). Overall we've spent a big packet and we now have a setup in defence that might get us to a good place.