Some gun just shoot, most don't. I just had the same kind of thing happen with a brand new T3 lite in 223. Just worked up for velocity with the components I had, some once fired and sized range brass I got off the EE (2000 of the buggers) and a 500 pack of bullets I got on discount. It shot every thing into the same small group size from mid to max loads. It was great! Then I tried some old half assed reloads in crooked PMC brass (the worst I have ever seen) with poorly thrown powder charges and OAL's all over the map. The damned rifle shot even smaller groups.
The real kicker is I have a reamer and bbl sitting here waiting for me to make a 223. But the day I went to the store to buy a Remington action for the build I ended up getting a deal on the Tikka. Sometimes life is to easy.