Not a clue. I applied for both at the same time - Starling may have been first actually, back in the very early days (when it was Mondo Beta!).
I actually found the email I sent to Starling and the attached screenshots at the time. They have have changed since then.
‘We are sorry. We are unable to verify your details and offer you an account.’ with a link to a ‘Helpful tips’ section. I thought it meant they needed more info, maybe a utility bill or something but when I got nothing more from them after a week or two I emailed to ask what was going on and got back
Even though Starling is in beta; as we are a regulated bank, we conduct identity, fraud prevention and credit checks to open a Starling account.
Based on the information you provided, we have done our checks, and are unfortunately unable to open an account for you at this time.
We understand you may like to know why; and we can say it is due to one or more matters arising from the identity, fraud prevention or credit checks we have conducted.
Then I realised it wasn’t just a ‘We’re sorry, we can’t offer you an account because we couldn’t complete your checks’, it was 'We’re done. Maybe that is obvious from the message they provided, but I thought it was a holding message while checks were continuing or waiting for something from me.
(Sorry to highjack this thread and turn it into a Starling issue, I did just want to highlight that all banks use the same sort of language - they can’t say too much about why they can’t offer you something).