Fulltext count is not the same as original count

[flancer64]

Fulltext results count can be more than original select results count (joinInner is used). Leave $this->_totalRecords empty to initialize later, after actual data have been loaded.

[ghost]

Opened internal task item MAGETWO-56721

[maghamed]

Fulltext results count can be more than original select results count (joinInner is used). Leave $this->_totalRecords empty to initialize later, after actual data have been loaded.

potentially it can. But in fact join

        $table = $temporaryStorage->storeApiDocuments($this->searchResult->getItems());

        $this->getSelect()->joinInner(
            [
                'search_result' => $table->getName(),
            ],
            'e.entity_id = search_result.' . TemporaryStorage::FIELD_ENTITY_ID,
            []
        );

is just a performance optimization for query
select * from catalog_product_flat where entity_id IN (list_of_10000_ids)
because inner join for temporary table which contains just IDs column with Primary Key on it is much faster than IN clause.

If we will apply additional filter(-s) using Collection Interface which will filter out some data - we will break Layered Navigation. And if we could re-calculate getTotalCount, it's impossible to make that for LN.
So, as for me it's okay to leave everything as is.