What's the difference between a PSA Graded 6, 7, 8, 9, and 10

PSA 10 1975 Topps #279 Milt May - https://youtube.com/shorts/Gr0boHoYFfA?feature=share PSA 9 1975 Topps #319 Johnny Oates - https://youtube.com/shorts/_vKIMMku3r4?feature=share PSA 8 1975 Topps #88 Tom Egan - https://youtube.com/shorts/61amegrUHjc?feature=share PSA 7 1975 Topps #649 Jack Heidermann - https://youtube.com/shorts/kwlFy6V4oGg?feature=share PSA 6 1975 Topps #341 Hal Breeden - https://youtube.com/shorts/OxETvg1_nBo?feature=share PSA 6 1975 Topps #448 Frank Duffy - https://youtube.com/shorts/kU7-CMmarJE?feature=share What makes these graded as they are? I ask because to my untrained eyes, the lower graded cards sometimes look as good as the GEM MINT 10 does. What am I missing?