In Multilingual Search IndexResource builds query to search engine (in my case Solr). Solr response returns nodes in all languages. Than createCollectionDataFromEntities grabs all data and returns only results in queried language but counter is set from solr response $primary_data->setTotalCount((int) $results->getResultCount());.
E.g. we have node in 10 languages. During blank query Solr returns all data in all languages (10 nodes). In search response there will be only 1 node but counter will be set as 10.
Furthermore because of limits during the query $query->range($pagination->getOffset(), $pagination->getSize()); it gets asked amount of results which and createCollectionDataFromEntities method and can return entities from first 50 Solr results and ignore others which are out of range but in response you can get 3 or 4 entities instead of real amount.
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I noticed this too but was able to resolve it by modifying the solr index settings.
Solr specific index options > Multilingual
Enable the setting: Limit to current content language. Reindex, and the count is correct now for the default language.