[Retric]

You can only focus on a relatively small number of different things at the exact same time.

  1 3 2 5 9 2 8 7

vs.

  1 3 2 5

  9 2 8 7

Memorizing the first sequence is trivial, but dealing with each half independently is much less mental effort. Now each idea can be more complex than just a number, but even still you make fewer mistakes by removing mental overhead.

PS: Now, their are a lot of tricks on how you can get better at this stuff. But if the parent poster tends to write bugs in most functions then simplifying things may help.