It can occur in these cases:

1. The default sorting of the Search Results Widget differs from Relevance. You can check out that in the Searchanise control panel > Search & Navigation > Search results widget section > Content tab > Default sorting option.
2. You’ve set up merchandising rules, so the promoted products are displayed at first. You can check out that in the Searchanise control panel > Merchandising & Promo > Merchandising section > Products or Categories tabs.
3. Products that match the search query are out-of-stock, and you’ve hidden such products from search in the Magento settings (Magento admin panel > Stores > Configuration > Catalog > Inventory > Stock options > Display Out of Stock Products option), or you’ve adjusted to show them at the end (the Searchanise control panel > Search & Navigation > Preferences section > Products tab > Show out of stock products at the end of results list option).
4. Some of the product fields are disabled and the app doesn’t search through them. You can check out that in the Searchanise control panel > Search & Navigation > Preferences section > Products Fields tab.