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 Smart Search Bar & Filters 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 Smart Search Bar & Filters control panel > Merchandising & Promo > Merchandising section > Products, Categories, or Tags tabs.
3. Products that match the search query are out-of-stock, and you’ve hidden such products from search via the Smart Search & Filter control panel (Search & Navigation > Preferences section > Products tab > Hide out-of-stock products option), or you’ve adjusted to show them at the end (the Smart Search & Filter 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 Smart Search Bar & Filters control panel > Search & Navigation > Preferences section > Products Fields tab.