Sessions vs. Reached Checkout for Headless site

[Colin5891]

We run a headless site, meaning that our shopify 'page' is only our checkout process - the rest of the website is on our own domain. 

With this structure, I would assume that to qualify as a 'session' on our Shopify page as per Shopify analytics, a customer would have to have initiated checkout. Basically sessions should = customers who entered our checkout process.

However, we fine that Shopify counts 2-3x sessions as it does 'reached checkout' events. 

With a headless structure, how could this be the case?