If two things are almost the same, but not quite identical, it looks like one of them is a mistake. Either make them exactly the same or make them noticeably different on purpose.
Having a consistent standard for things like hem depths, binding widths, and button sizes keeps it easy for you, your factory, and your customers.
For you, it simplifies sourcing and inventory. You can use the same trim or notion across multiple styles. On the technical side, you build up a set of standards for your brand that streamline new development and create quality across your collection.
For your factory, they can set up their machines and attachments to sew those specific finishes and not have to re-adjust as often. This means fewer mistakes and more potential cost-savings.
For your customers, even if it is subconscious, they will get used to those same details and start to associate those with your brand. It is a subtle, yet simple way to build trust and reinforce your brand’s unique identity over time.