BGS is a 20-point scale, PSA is currently about an 18-point scale (there is no 0.5 or 9.5, I think), and they only started doing half-points a few years ago. Plus, BGS raised the bar with their 10s being very difficult to score when they started up, generally viewed as indisputably mint cards. PSA standards were somewhat more broad, as indicated by the grade descriptions.