I haven't tested my 1RM on the SSB or the squat recently, but my work sets are all about 10-20% lighter.
My most recent heavy squat set was 405x9. I've been doing SSB for the last 6 weeks and my most recent top set was 375x6 with the SSB. Both done under the same conditions with just knee sleeves and a belt.
I think my 1RM's would be a little closer, but that's just a guess. I did an easy 425 but nothing heavier on the SSB. If I had to guess I could probably get 475 on the SSB and 530 or so on the squat if I were to max today.
Your squat should definitely be higher. My guess is technique and probably some mindset. The SSB is probably easier to walk up and squat with, whereas the back squat will involve getting comfortable in an uncomfortable position and putting more work into your technique.