The result of the GET request will be a number of customers more than the customers the will be shown in the Shopify admin and present in that saved search.
This issue will exists only if the saved search have language filter, in others condition it will work.
Thanks for sharing your concern with us here in the Shopify Community.
As this relates to the use of the Shopify API, I have just moved your query here to our API forums.
This is the best place to get visibility on anything relating to the API, as our developers and Partners monitor and respond to these threads.
That said, in a case where you believe you have encountered a bug or other issue breaking the API (rather than queries about how to use the API) we would ask you to get in touch via our Help Centre here to report this info.
There we’ll be able to escalate this report to our technical team internally, who will be able to determine if there is indeed a bug or other problem present.
Thanks again for taking the time to let us know about this!