Lightning Experience Global Search combines standard search with type-ahead (instant) search, which functions like appending a wildcard (*) to the search term. Classic does not include instant results, so fewer records appear for the same query.
When you search for a term in Lightning, you may see more results than when you perform the same search in the Salesforce Classic UI.
This is expected behavior. Global Search in Lightning Experience is designed in a way that includes standard search results as well as the instant search results you see when you begin typing into the search bar.
'Type ahead' search is more complex than this, but the functionality is similar to performing a standard global search with a wildcard (*) appended to the end of the search term. The combination of the global search, with the addition of the type ahead records, may result in more records being returned in Lightning Experience versus Classic.
To learn more about the 'Type Ahead' function, please review the 'Instant Results in Lightning Experience' section of the What Are the Results I See As I Type in the Search Bar? documentation.
To effectively receive the same results for a search term in Classic and Lightning, users may consider manually adding a wildcard (*) onto the end of their search term(s) in Classic.
For example, search for Smith* instead of Smith.
Why Am I Seeing Search Results for Similar Terms?
Search: What’s Different or Not Available in Lightning Experience
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