Hi @charlotte61, You pose an interesting question. My guess is both conditions are related but that PMR doesn't progress to GCA. It's just my non medical opinion that there are other factors involved that determine whether or not you have or will develop GCA. Here's some related information on the topic from a 2017 article.

"Although 40% to 60% of patients with GCA have associated PMR symptoms at diagnosis,⁶ only 10% to 20% of patients with isolated PMR will develop symptoms of GCA."
--- Latest Advances in the Diagnosis and Treatment of Polymyalgia Rheumatica: https://www.medcentral.com/pain/chronic/latest-advances-diagnosis-treatment-polymyalgia-rheumatica.

I've had PMR twice and it never progressed to GCA although my doctor always had me watch for GCA symptoms.