When does iOS updates download over cellular?

I was just informed that we were over our monthly data usage. I checked and my cellular usage shows 5.5GB for a software update.

I have always had my Software Update set to Download Only and specifically on the three settings:

Automatically Install

IOS Updates: Off

Security Responses: ON

Automatically Download

iOS downloads: ON

With those I've never seen a cellular data usage for an iOS Updates. Maybe I've been lucky and that last setting should be off but I thought that setting would only use Wifi.

Thank you for any info/recommendations.