size 37 of Men's Katana Lace will give size 37 of Men's Skwama, but size 37.5 of Katana will give size 36.5 of Skwama... You can find this by these two links.
https://sizesquirrel.com/public_match?want_item_id=323&have_item_id=63&size=37
https://sizesquirrel.com/public_match?want_item_id=323&have_item_id=63&size=37.5

I love your website! I believe you have all the data necessary to build a great recommender system. However, if I understand correctly, you are not leveraging machine learning to make your recommendations. Please correct me if I am wrong.

I am a Data Scientist and build ML models for a living and have worked on fit/size prediction in one of my previous jobs. I would love to give a shot at creating a ML recommender system for your website. I believe this would avoid scenarios where you get a response such as 'We're unsure which of the sizes above will work.' and many 'size not found'

For a first prototype, I would just need a relatively simple data dump for a first version:
anonymized_user_id | shoe_model/sku_id | shoe_size that's about it! We can look together how the prototype performs offline and, if you feel like it, consider pursuing a more sophisticated version that takes into account foot shape etc...

Please let me know what you think!