Strength combines Stamina (left) and Muscle (right). Starting fighters receive a d4 bonus in muscle, not in stamina, and they may not surpass their racial strength maximum in either. For example, a human with a 20 strength before rolling the d4 bonus, would still cap at a 23 muscle score.
This is a companion discussion topic for the original entry at http://www.athas.org/articles/ur-braxa-house-stats