Confusion about the use of active and passive voice

Question

Here is a sentence from a textbook:

「どちら／どっち」は、二つのものから一つを選択するときか、2人の中から１人を選択するときに使われます。

Passive voice makes sense here because the actor is irrelevant, but then there's this sentence on the same page:

「どの＋N」は三つ以上のものから一つを選択するとき、または、３人以上から1人を選択するときに使います。

These two sentences have nearly identical structures but for some reason the latter is in active voice. The only explanation I can come up with is that this is done to avoid repetition.

My questions are as follows:

1. Why did the author switch between active and passive voice?
2. How does the meaning change?
3. Who is the implied subject in the active sentence? "People"?

Answer 1

My answers to your questions are as follows:

1. God knows. I can't speak for the author. They may have wanted to avoid repetition out of personal preference, may have been just following the house style (if there was any), or may have just written it that way without thinking about it at all. What I can say, however, is that they definitely could do it and did it without sounding strange and affecting readability for me. (Well, I didn't read the whole page, but.)

2. Beyond the usual change in literal meaning the active-passive alteration produces (which isn't much of a change in meaning), not much to speak of, I assume.

3. Again, I wouldn't presume to put words into the author's mouth, but I'd wager that they didn't have any particular grammatical subject in mind, not even an implied one. When reading the first sentence, it probably didn't occur to you to wonder by whom 「どちら／どっち」 is used in such a way as described, even though there must be some entities who use it, and the author could have specified who they are. And you didn't have any trouble understanding the meaning and the message of the sentence, right? The same with the second one, except that now that same missing information would be expressed in subject position. But in Japanese you don't need a grammatical subject any more than you need a by XXX phrase in a passive sentence unless there is a special need. If you are talking about translating the sentence, the generic we you, one and what have you will do the trick.