can make a big difference in a load. Black powder, generally speaking, isn't quite as touchy on small differences in a load(a grain or less). But with smokeless propellant, in a lot of firearms, just a fraction of a grain can make a big difference in accuracy.
Case in point: once worked up a load for .303 British using Hornady 150-grain softpoints. Started at the low end of the load range using IMR4064 powder, and got about 3" groups at 100 yards. Upped it by a half-grain, and a little better. At one grain higher the groups shrunk to 1.5-2" at that range. That last was close to two grains below the suggested maximum, and gave by far the best accuracy with that powder/bullet combination.
Went to the range tonight to try out a couple of things, one of them being a cast bullet load for the M1 Carbine. Again, started at the low end, went up by .2 grain increments. The first two gave about 2" groups at 30 yards; up it another .2 grain and I got one big ragged hole with one about a half-inch outside that. Another .2 grain, the same. Add .2 more and it opened back up to about 2". Guess which I'll be trying out at longer ranges?
Cases, powders, primers, bullets; they're all different. Usually. I've got an old 8mm Mauser I've mentioned before that doesn't shoot badly with any load I've tried, but with the one it really likes I consistently get 1" groups at 100. I've fired others that would only shoot well with certain loads; get just a little outside the parameters they liked and it started patterning instead of grouping.
There are times I think Kim is right to call handloading 'alchemy'; there've been times I'd swear that breathing the right way or muttering under your breath(in the right frame of mind, of course) made a difference in how a load worked out.