I have a demo store for my theme - https://shella-fashion.myshopify.com/
The Online store speed is 59 - https://take.ms/A2gry
When I check insights per home, product, and collection pages. I saw other numbers
Home page - 76 mobile, 95 desktop - https://take.ms/XelTg
Product page - 73 mobile, 95 desktop - https://take.ms/gtZ4F
Collection page - 74 mobile, 87 desktop - https://take.ms/hPHrg
Why Shopify shows only 59?
PageSpeed Insights and Shopify's Speed Report are both based on Lighthouse, but they run their tests independently of each other. PSI's hardware might be different from Shopify's hardware for testing, or they might be using different configurations for Lighthouse, or it could be any other difference in environments. I have my own system that runs Lighthouse tests for our clients and I routinely see widely different scores from PSI. It's best to view the score and metrics these tools provide you as very rough estimates.
I believe Shopify explained clearly here.
"Because the test conditions are different when run from Google PageSpeed Insights, your score might be different from the score listed in the Online store speed report."