Editing growths in Nightmare for FE4 is leading to confusing results

[Slig]

I was using Nightmare to increase most of the first generation Genealogy characters' growth rates to exactly 100%. Characters with holy blood had their bonuses taken into account when setting their base growth rates, so Lewyn for example only had a base speed growth of 40% because his holy blood should account for the extra 60%. If a character had a vanilla HP growth over 100% I left it at its normal value. 

While everyone did have clearly inflated growths, they for some reason weren't flawless and they would often miss one or two stats on level up, with Sigurd once even failing to increase HP despite his 110% growth rate. Would any of you know why this is happening? 

[Lightcosmo]

FE4 always has this issue, Ulster can fail to get HP on level ups despite having around 170 percent or so with the right parent.

[Ertrick36]

Are the "bugged level-ups" always happening in the arena?  It's a well-known issue (at least well-known among LTCers, hackers, and others more knowledgeable about the game) that the arena screws with characters' growth rates and seems to always make them worse.  Theories abound for why this happens - maybe it's the way RNs are rolled, maybe it's a failsafe to try to make the arena less OP (yeah, right).  But the arena has been known to render even 100% growth rates inert sometimes, and no one knows why that is.

I'm not sure if the problem ever occurs outside of the arena, but people definitely noticed it most in the arena.

[Slig]

I'm not 100% sure but the only imperfect levels I noticed were in the arena where they were quite common. The first few levels I got during the prologue when I was paying closer attention were all perfect.