If not - change a bank.
While that's an interesting data point, plausible explanations could be anything from very positive, such as
(a) Stripe's Checkout process being dramatically more effective for signed-in users, for example because of the reduced effort required to complete the transaction
to very negative, such as
(b) Stripe's Checkout process causing a horrible drop in conversions for users who aren't signed in, for example because of the added complexity that comes from their "remember me" mechanism or the unfamiliar branding.
It doesn't seem at all surprising that previous Stripe users would convert better with either of these relatively extreme explanations -- either those people had an easier interface to use, or they were necessarily already familiar with the Stripe brand.
With the greatest respect, while that may be true on balance over the entire population of Stripe-using businesses, it won't necessarily be true for all of them individually.
(I have inside knowledge of various businesses, at least one of which did switch from Checkout to Stripe.js for the kinds of reasons I mentioned before. However, I have no idea how well or otherwise any mostly small and UK-based companies I know about might represent your customer base as a whole, so I didn't want to be unfairly critical.)
You can see even in Stripe's earliest UX the desire to dramatically limit the "pain" that goes into entering CC details online.
When they added the "remember me" button I said to myself this is going to wind up being the "billion dollar checkbox" once merchants really understand the power of not forcing people to re-enter their CC details.
It's still somewhat mind-boggling to me how badly browser "auto-fill" solutions fail at this incredibly valuable problem.
Amazon and Apple, IMO, basically hold onto their dominating positions in physical & digital content respectively because their hold so many CC's and it's just so much damn easier to not have to re-authorize, re-input, etc etc.