In his autobiography, Hunt describes the young Rossetti's "southern" (i.e. Italian) features, with "grey eyes, looking directly only when arrested by external interest".
It is likely that Rossetti was constantly looking up at Hunt from work on his reciprocal portrait drawing (now at Birmingham), which explains the slightly foreshortened angle of the head.
[http://hoocher.com/William_Holman_Hunt/William_Holman_Hunt.htm]