On separatedbyacommonlanguage.blogspot.it¹ (Observations on British and American English by an American linguist in the UK) we can read the following explanation:
The translation problem in just about isn't just about just.
Let's think about about. The (UK) Collins
English Dictionary gives us this sense-definition, which is not to
be found in the American Heritage Dictionary or Merriam-Webster:
13. used in informal phrases to indicate understatement : I've had just about enough of your insults it's about time you stopped
Aha, the famous British understatement. Rather than saying I've had
enough, you put an about in to soften the blow. And then a just to
soften it more.
For more details, see the site I mentioned, which seems quite well done.
¹ Probably, from other countries, you have to change 'it'.