I've been doing some ammo testing myself lately, with some interesting results.
I bought a Tikka T1x about a month ago, and have been trying the ammo I had on hand from my CZ 452 Varmint.
The CZ showed good results with RWS Target Rifle, SK Pistol Match and Lapua Center X.
The Tikka, in contrast, shot the RWS TR poorly, the SK PM was a bit better, but the Center X was reminiscent of a shotgun pattern.
The best ammo through the Tikka so far has been CCI SV, SK Biathlon Sport, and Eley Force HV of all things.
My early-in-the-game results seem to indicate that the Tikka prefers higher velocity ammo more than than the CZ does.
I hadn't really considered velocity nodes to be a significant factor in rimfire ammunition, if it proves to be the case it'll be a bit of a breakthrough for me.
It does make the whole lot testing regime make a bit more sense though. No one is going to convince me that Eley, RWS and Lapua deliberately make some lots "better" than others, or that quality control is that bad on true match ammo.
I might believe that they deliberately make some lots slightly faster or slower than others, knowing full well that by doing this there's a better chance of at least one lot being outstanding in a given rifle.
There might be other subtle differences, such as seating depth, crimp force, etc.
My plan now is to run the different types of ammo over a chronograph to try to find a correlation between velocity and accuracy in that rifle.
The trouble with HV ammo is that most of it is designed for hunting or simply to cycle semi-auto pistols and rifles, with accuracy being a secondary concern.
Some of it is sketchy enough that you can hear that one round is supersonic and the next isn't, American Eagle for instance. I shot some of it through a Marlin semi-auto rifle and gave up on it for that reason. Almost as bad as Winchester Wildcat.
Testing the Eley HV and some of the SK/Lapua HV offerings over a chrono will be interesting to see how good it actually is.
The CCI SV at an advertised 1080 fps has been a pleasant surprise at $4.00 per box. It shouldn't be that consistent compared to ammo costing 4 times as much, and the next batch may prove that.
In perspective, I think the Center X I have will shoot consistently poorly through my Tikka, while the CCI may be hot and cold, as the OP saw in his rifle.
The last time I ran CCI SV over a chrono through the CZ the results were surprisingly good, as I recall. Not so much accuracy as extreme spread and standard deviation, the CZ shot it into decent but not outstanding groups, however the velocity numbers were pretty good.
If I could get that performance reliably, it'd be a hel* of a sleeper.