95 burns slower, that's one of the ways it increases knock resistance (same effect as retarding the ignition). You may find that a bike built for 91 prefers the faster burning fuel at lower speeds, probably because it has the timing set for it.
Dunno 'bout your non-DR, my non-DR runs 11.5:1 compression ratio and demands 95. The DR is only 9.5:1 or something, a shitload less.
My Triumph had 10.6:1 CR and was spec'd for 95 RON. It'd run like pigs arse on 91, bl00dy appalling. Later, even with high-comp pistons and a head job pushing the CR to nearly 14:1, there wasn't really any difference between 95 and 98. But it did have wicked wild cams in it: high lift, long duration and lots of overlap. So the effective dynamic CR was less than the mathematical 14. I ran it on 98 whenever I could "just in case", but I never ever noticed or measured any difference.
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