My brain translates it from: "su propio" -> "it's own".

Don't ask me why, thanks for the correction

Spanish only has two genders, masculine and feminine, so these sorts of mistakes are an interesting revelation of how language works.

My native language in turn doesn't have articles, so people frequently put "a" when there should be "the" and vice versa, or put it where it is normally omitted.

Some features of languages are hard and unless one acquired an intuition early in life, they'll keep making mistakes.

I know. I’m a native Spanish speaker.