I agree with @Taylor Clark , but just to add a bit more context from some experience we have had working with small and large merchants.
Inbox providers (like Gmail, Yahoo, Outlook) all have different algorithms to determine whether your email lands in their primary inbox, a filtered folder (promotions tab or other filters), or the Spam folder - aka deliverability.
These algorithms have factors specific to the email you send, the history of all emails you send, and the history of that specific user, and the aggregate of users who receive that same email and past emails. It wouldn’t be too far fetched to think that they can even use external factors - like how long people spend reading the email, if they clicked on the link how long did they come back to their inbox, and if you are really cynical, Google especially, probably even knows things you do beyond the email! (Searches after reading email? Website powered by Google Analytics? Purchase receipt email after a click? Are you using an Android Phone?) Ok, that got dark pretty quick…
Long story short, larger brands and merchants can get away with a lot because of the history and “reputation” that they have earned (or destroyed) over a larger volume of emails and possibly time. As a younger brand, or a lower volume merchant, I try to advocate for Klaviyo’s deliverability best practice even though it may contradict to what the “big brands” are doing.
On a completely side note, I always remind people to use their brand’s “smallness” to their advantage and try more things. Big brands are stuck in their highly styled, polished and sometimes repetitive templates - you have the freedom to experiment without a committee of approvals.
One thing that has surprisingly worked well are emails that are plain text only, written like an email you send to a friend (clutch your pearls, like savages!). I’m not sure if it’s the deliverability algorithms, or just the “authenticity” feels, but I’m always surprised at how well it performs compared to emails that takes a whole design team, a fancy photo shoot, photo touch up artists, and the blessings from legal, the brand police, and which side of the bed the CMO woke up on to produce!