"Just" in this context is an intensifier. It makes the sentence a little bit stronger. But in practical usage, they have the same meaning. The difference is similar to: It is so hot this summer. It is just so hot this summer. Or: It is so nice to meet you. It is just so nice to meet you. In all cases, the word "just" makes the statement a little bit stronger.

Could = Can in this case. "Could" is more formal than "can".

the difference is tone of voice ,"just" have a bit stronger.

Can I ask you that why you use "could" here: Could there be any difference between these idioms? thank you.

Hmm. I don't know if there is much of a difference between the two. Both of them are used to add to the irony of a situation; for example: "I had to pay for my ice cream and, wouldn't you know it [or wouldn't you just know it], I left my wallet at home and had to ask my 5 year old to pay for the both of us with her allowance." I would say that wouldn't you just know it sounds more awkward. Hope this helps! Good luck.