Keltham isn't thinking that Carissa is deceptively faking her sexual responses; it's not someplace his thoughts would readily jump. He is distracted, among other things, by the pattern of red trails he's managed to leave on Carissa's back, though his hardest-raked fingernails haven't made her bleed. She is, again, right about him; he likes seeing her annotated.
Keltham is noticing that Chelish women's sexual responses don't make sense in terms of the patterns he's been trained for; or at least, Carissa's patterns of muscle tenses and breathing aren't matching up well with Carissa's overt sounds, and he's not sure which one he ought to believe. Maybe there's some kind of Permanent Cheerfulness thing going on, like with the research harem in his lessons? If Carissa had spent a lot of time doing something like that during overcomplicated romances with no off switch, she wouldn't necessarily be able to stop, especially if she was otherwise distracted by fingernails. He does not wish to stop and talk about it either.
Keltham decides he will mostly believe in Carissa's less overt responses for now, and starts experimenting with standard-variations-over optimal stimulation patterns, to see if he can hill-climb to anything that produces a readably different less-overt sexual response, or if he just can't read her at all.