The jat-reflex rules are not without exception. For example, when short jat is preceded by r, in most Ijekavian dialects developed into /re/ or, occasionally, /ri/. The prefix prě- ("trans-, over-") when long became pre- in eastern Ijekavian dialects but to prije- in western dialects; in Ikavian pronunciation, it also evolved into pre- or prije- due to potential ambiguity with pri- ("approach, come close to"). For verbs that had -ěti in their infinitive, the past participle ending -ěl evolved into -io in Ijekavian Neoštokavian.
Why did re or ri develop instead of pre?