That clumping emulates delayed action that would be extremely difficult to achieve directly coding into the game otherwise. If 2 more characters are adjacent to each other in the initiative a player can alternate partial moves between them to achieve a coordinated effort. For me the standardization to the D20 did not occur until 3rd edition, so I may be more inclined to accept the variance. While your example is technically correct, look at this way. Larian knows the available options. This makes it easier for Larian to provide encounters in manner they intend. This is not just to make certain mobs more likely to react first either. I point to the Ancient Forge Encounter which by design was created for the automaton reacts last and plays into my first observation.
So, I reiterate. The alterations change the dynamics of play, compared to standard. But such changes do not break the game and (here is opinion) falls in line with the only answer we ever receive from Larian (or from any other game developer) about the changes.
