You are looking at the numbers and not the meaning of the individual companies grades.
PSA 10 = Gem Mint
BGS 9.5 = Gem Mint
BGS 10 = Pristine
PSA do not have a pristine grade. If you can tell the difference between a BGS 9.5 and a BGS 10 you are doing pretty well. I have seen two of the same cards - one BGS 9.5 and one BGS 10 and I cannot tell the difference under magnification. Part of me thinks its the **** value, but hey that's me.
I have submitted a number of BGS cards cracked out of their cases to PSA. I have had BGS 9's come back PSA 9. I have had BGS 9.5 come back PSA 9. I have also had a BGS 9.5 come back a PSA 10, but I have also had a BGS 9 come back a PSA 8. I have submitted to both grading companies, and settled on PSA purely because I believe they are more consistent with their grades. I can pretty much tell what grade cards are going to come back from PSA with - when submitting to BGS it was pot luck. Not saying PSA are perfect - they aren't. Just that they seem to get it right more often.
Also don't forget to ask Chad about his Larry Bird Auto he sent to BGS
This is purely my opinion and observation from actually submitting cards.