> +4.5pp for software engineering is suspiciously low compared to +51.9pp for healthcare.

This stood out for me as well. I do think that LLMs have a lot of training data on software engineering topics and that perhaps explains the large discrepancy. My experience has been that if I am working with a software library or tool that is very new or not commonly used, skills really shine there. Example: Adobe React Spectrum UI library. Without skills, Opus 4.6 produces utter garbage when trying to use this library. With properly curated/created skills, it shines. Massive difference.