This study explored possible reasons why IELTS candidates usually score low in writing by investigating the effects of two different test designs and scoring criteria on Iranian IELTS candidates' obtained grades in IELTS and World Englishes (WEs) essay writing tests. To this end, first, a WEs essay writing test was preliminarily designed. Then, 17 Iranian IELTS candidates wrote two essays on the same topic, one under the IELTS test condition and one under the WEs test condition. Each of the 34 obtained essays was scored six times, three times based on IELTS scoring criteria, each time by a different rater, and then, three times based on WEs scoring criteria. The results of repeated-measures ANOVA showed that test design and scoring criteria had significant effects on essay grades. The study concludes that some of the reasons why IELTS candidates usually score low in writing may be rooted in the test design and scoring criteria of the IELTS essay writing test, not necessarily in IELTS candidates' weaknesses in writing. The implications of the study focus on the importance and relevance of the results to IELTS candidates, international students, and the future of assessing writing in World Englishes contexts.