Welcome to Community Loaves. It sounds like you know your way around your levain. The Purist definitely favors a slow bulk. There could be a difference based on flour alone, simply because the fine bread flour is milled so “freshly”, it could be more enzymatically active than the Bob’s Red Mill. I’m not confident that will explain the full difference. Thankfully you will have a comparison to provide at the end of the next week. I’d love to hear a follow-up report.