It Was Decided, I Decided ことになる・ことにする

ことになりました almost always implies the decision came from outside the speaker, a company transfer, a school assignment, a circumstance that worked out this way, rather than a personal choice freely made.

ことにしました reports a one-time decision, “I decided to.” ことにしています, built with ている, reports an ongoing personal rule or habit you’ve committed to, “I make it a policy to,” a subtle but useful shift in nuance.

ことになっている, with ている on ことになる, describes a standing rule or schedule that’s already in place, often a workplace policy or an arrangement everyone’s expected to follow, rather than a single decision made once.