Human psychometric questionnaires are increasingly used to characterize large language models (LLMs), but it remains unclear what exactly their responses reflect. To answer this question we analyze responses from 50 LLMs across 45 questionnaires spanning multiple psychological domains using psychometric methodology. We found a dominant principal component explaining 47.1% of variance across standardized, response-derived questionnaire-level primary scores, revealing substantial shared structure across instruments. We investigate its meaning through semantic analysis, respondent framing, and human annotations. Semantic analysis suggests that the component partly reflects experiential self-attribution: how readily models endorse statements describing subjective states and bodily experiences. A model score emphasizing items whose disagreement decreases under human simulation correlates strongly with the component ($r=0.864$). Human ratings of 289 matched items show that inner-life content is positively associated, and social desirability negatively associated, with item-component correlations after mutual adjustment and controls for other semantic properties. Together, six rated properties explain 22.1% of variance in Fisher-transformed item-component correlations, supporting a partial interpretation involving both experiential self-attribution and socially desirable responding. These findings suggest that LLM psychometric profiles share response tendencies that require interpretation beyond the human traits their questionnaires were designed to measure.

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