To my eye, everything is different in those two images, not just the specular. The whole image is lighter, apart from the background.

Perhaps it's to do with the fact that Poser is 'combining' everything in linear colour space and then gamma correcting, whereas in Photoshop you are combining individually gamma-corrected passes.

Possibly?

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)