Due to differences in the systems used between the API provided by Rakuten Webservice and the search results on the Rakuten Ichiba website, it is not possible to achieve a complete match.
However, it is possible to make the search results on the Rakuten Ichiba website closer to the API results through configuration.
In the item search API, the input parameters "field" and "availability" are set as follows by default.
availability=> 1: Only available products
field=> 1: Restricted search (prefer fewer matches with the same keyword)
Therefore, setting both values to "0" will bring you closer to the search results on the Web site search page.
We would appreciate your kind understanding.