A mismatch between the admin preview and the live storefront is almost always explained by one of three things.
Preview uses your unsaved settings
The preview scores against whatever is currently on screen in the admin — including changes you have not yet saved or published. The live store always uses the published version.
This is the most common explanation. If you have made changes in the admin without saving, the preview and live store will produce different results.
Your live store may be remembering a previous answer
Shopper answers are stored for 30 days. Once someone has reached a result, the recommender re-runs automatically on their next visit and shows a size before they enter anything.
Test in a private browsing window to see the true first-time experience.
Check that you are comparing the same product
A different product may match a different size chart, or carry different product information. Make sure the product you are previewing in the admin is the same one you are checking on the live store.