I'm formatting a bulleted list of benefits covered by a dental plan. I believe the text originally came from the insurance company's official "schedule of benefits" document, which is written in legalese. A bunch of the benefits are limited to "once each" some number of months, as in:
- Bitewing X-rays once each six months
- Study models and casts used in planning treatment once each 60 months
- Periodontal surgery once per quadrant each 36 months
That sounds quite strange to me. Is there a technical distinction between "once each n months" and "once every n months?" I think the need for legal precision — or a fear of legal imprecision — is the most plausible reason why someone might choose the unusual phrase "once each" over the more common "once every" or "once per," but I am not a lawyer.