Why there are days between "create" and "trial start day"

I'm using pro site and my trial days setting is strange. My trial days is "3" days and create date is "12/13". So trial start day should be "12/13" but receipt show trial start day is "12/15" and start payment day is "12/18".
Why there are "3" days between "create" and "trial start day"? Please check the screenshot attached in chat for my PayPal receipt.