Sie setzen DISTINCT IN sum( rating.rating) as total_rating, deshalb das Ergebnis (12=17-5 ), da es 5 nur einmal bei der Berechnung der Summe enthält.
select review.comments, review.user_id, count(distinct rating.id) as rating_count,
sum( rating.rating) as total_rating from users
left join review on users.id = review.user_id and review.shop_id='1'
left join rating on users.id = rating.user_id and rating.shop_id='1'
where review.shop_id='1' or rating.shop_id='1'
group by users.id, review.user_id, rating.user_id, review.id
Hier ist SQLFiddle
Beispielausgabe: 
Hoffe das hilft
