We do not begin with punishment - בפורענותא לא מתחילינן
Summary:
למ"ד איוב בימי משה היה he was placed after תהלים since תהלים is the conclusion of רות. If we maintain
איוב מעולי הגולה היה, he was placed by תהלים since תהלים איוב and משלי are of a similar syntax (and we did not want to interrupt the works of שלמה) Alternately איוב and תהלים had many contributors.
A punishment for which there is an end - פורענות דאית ביה אחרית
Summary:
The אחרית of רות was for כל ישראל while the אחרית of איוב was only personal. Alternately the intent of the שכר was to relieve him of his עוה"ב. Therefore it is not a good אחרית.