Today’s idiomatic expression rather falls under the category of typical English speech patterns because it consists of two parts – “It’s not that… it’s just that…” – and provides a blueprint for the whole sentence!
It’s funny, but I haven’t heard this concept of typical English speech patterns anywhere else so I’d like to claim full copyright for coining this term and also using it. It’s a joke, of course, but on a serious note –
…it’s very important to learn such and similar speech patterns because that’s how you become capable of speaking without much thinking about HOW you’re going to wrap up your thoughts ❗
When you do some research on English sentence structures, all info is centered around syntax – simple sentences, compound sentences and so on. Does that type of information help us to speak more fluently?
All it does is facilities our analytic facility which is in fact detrimental to our oral fluency!
Don’t believe it? Please read this blog post where I’m clearly illustrating what happens in a foreigner’s mind when they’re overwhelmed by analyzing everything they want to say which is a direct result of extensive grammar studies!
The phrase “It’s not that… it’s just that…”, on the other hand, provides a very practical blueprint for a sentence, and all you need to do is fill in blanks with the respective concepts and words (collocations and expressions, of course, also come in handy to help you with that!).
You don’t need to categorize it, you don’t need to analyze it.
Just take it for what it is – a natural English speech pattern that can be memorized and used in your daily English conversations! 😉