Thank you for your answer.
I changed the value to full width (https://prnt.sc/0PSUoktqapRr) and the page width is still different (slightly narrower) for the Checkout subpage.
– Checkout -> https://prnt.sc/1brQ3zEvqLj1
– Another subpage -> https://prnt.sc/VmrNFYiNcBzd
For some reason, the Order subpage is slightly smaller in width.