You mean 5% lower velocity, and max. damage range is extended by 5 meters. It is within that 5 meter span that you require one less bullet to kill versus a 1000 hp target. You may also require one bullet less to kill on targets with less or more than 1000 hp, the idea is that an integer number of max. damage shots result in a kill, but an integer number of (max-1) damage shots do not. You may also sometimes benefit from SPA beyond the maximum damage range, as the damage is slightly better all the way up to the minimum damage range; pushing the max. damage range 5 meters further tightens the range where damage falloff occurs, so at any given distance up to the minimum damage range, you'll do slightly more damage with SPA.