A consulta complexa do MySQL está dando resultados incorretos

select * from (
    select a.idArticle, a.content, max(`order`) max_order
    from words_learned wl
    join words w on w.idwords = wl.idwords
    join article a on a.idArticle = w.idArticle
    where wl.userId = 4
    group by a.idArticle
) a
left join (
    select count(*) unknown_count, w2.idArticle from words w2
    left join words_learned wl2 on wl2.idwords = w2.idwords
    and wl2.userId =  4
    where wl2.idwords is null
    group by w2.idArticle
) unknown_counts on unknown_counts.idArticle = a.idArticle
where unknown_count is null or unknown_count < 10
order by max_order desc
limit 100