Why does the portrait of Henschell shock Conway? (chapter 7)

Henschell would have been an older man and his death, and yet, the prortrait, done just before he was killed was that of a young man.

He stumbled to his feet and strode across to the trembling circle of light. The sketch was small, hardly more than a miniature in colored inks, but the artist had contrived to give the flesh tones a waxwork delicacy of texture. The features were of great beauty, almost girlish in modeling, and Conway found in their winsomeness a curiously personal appeal, even across the barriers of time, death, and artifice. But the strangest thing of all was one that he realized only after his first gasp of admiration: the face was that of a young man.