^^This^^^Probably reduced capacity and small rifle primer
Cap... My issue was with the Lapua 6.5 CM brass and RL26. The lapua 6.5 CM's smaller case capacity kinda hampers velocity with RL26. I was getting better speed with Hornady brass and RL26 just because I could get more powder in the case before powder was too compressed.
Also, not only does the Lapua 6.5 CM bass use a small rifle primer, it uses a smaller than normal flash hole as well. Even with a CCI450 Mag primer, I would get erratic SD with RL26. Went to the Hornady case and all those issues went away.
Now I use H4350 with the Lapua 6.5 CM case and that works great.