I thought I was going to solve this puzzle, as I was once told it correlated to whether the clincher was played at home or on the road. When we clinch at home (like this year), NBA is written in green in a white circle with small green marks at the top and bottom. When we clinch on the road (like in 81), NBA is written in white in a green circle.
But looking over the banners, that's not true. We clinched at home in 57, 60, 61, 62, 64, 65, 66, 84 and 86. But 64 and 57, for example, clearly have different logos.
Maybe there really is no answer.