Same word - written with kana and kanji in two places in the same paragraph. Why?

Question

I am aware that some Japanese words can be written in either kana or kanji and that the rules about it are not set in stone. This has already been discussed in some questions and answers here (e.g. Usage of kanji for words usually written in kana) but my question is more specific.

My question is about why sometimes the same word would appear once written in kanji and once written in kana very close to each other. For example, in Haruki Murakami's story I'm reading now (「かえるくん、東京を救う」), there's two different way of spelling of 何/なに and 僕/ぼく within the same short paragraph. This is a continuous utterance by the same one character in the story.

みみずくんがその暗い頭の中で何を考えているのか[...]

[...]彼はなにも考えていないのだと僕は推測します。

ぼくには説明のつけられないことです。

Why is it spelled differently like that? Does it have any meaning? Is it just style? The sentences with 「考えている」 are very similar to each other, yet 何/なに is spelled differently.

The same difference in spelling is not limited to those words or this paragraph. It happens throughout the story and affects different words.

UPDATE

I feel that the answers below, while helpful, didn't really provide the ultimate answer to my question (or maybe such an answer just doesn't exist). I especially feel that concentrating on ぼく written with kana to mean childish language is not on the mark as the context of the story doesn't suggest that (I don't blame the helpful people who answered the question for that - they don't know this context).

I can also add an example from another story 「蜂蜜パイ」 from the same collection of short stories. Again two different ways of spelling are in the same passage, close to each other, spoken by the narrator this time.

[...]今度は左手を袖の中にひっこめた。
それはすぐに袖の中に引っ込められ、[...]

Answer 1

People normally use consistent way of writing. I would say it's his style to mix 僕 and ぼく. When you use ぼく instead of 僕, the meaning doesn't change; however, the impression it gives to the reader is slightly different. It gives a somewhat childish, soft impression because hiragana is used. 「きみ」 sounds a bit more tender than 「君」 for the same reason. I guess you could say so for なに/何 also, but for words other than first person, second person, or third person, I think there's not much difference in writing in kana and kanji.

This may not be relevant, but another situation where ぼく and 僕 are distinguished is when talking to a little boy. 「ぼく」, 「わたし」 are sometimes used as "you" when talking to a child.

ぼく、迷子？

is asking "Are you lost, boy?" but you wouldn't (at least I wouldn't) use

僕、迷子？

since 僕 has a more mature impression than ぼく.

Answer 2

I can think of two reasons. As stated by @NigoroJr, when children's language is being used, it is usually spelled with kana. For instance, in よつばと, everything the adults say is written in kanji, but everything Yotsuba (a four year old girl) says is written in kana (and bolded ones at that).

The other thing is that, in certain grammatical forms, words are written with kana instead of Kanji. One I see a lot is 欲しい. When being used as an adjective, it's written with Kanji, but when you're using it in the 〜てほしい form, people usually write it with kana. My experience has been, if a word is stand-alone, it's written with Kanji, but when it's part of an expression or a grammatical function, it's written in kana to distinguish it/make it easier to write.