What do you think is the meaning of "just so" here? "merely so" or "exactly so"?
Since a reinforcer must serve to strengthen the desired response, it is imperative that the counselor select a reinforcer that will have precisely this effect on the subject. For instance, the counseleor wants John and Ellen to do certain tasks. To encourage each one, John is offered a ticket to a football game and Ellen is offered a ticket to the ballet as a reward for performing the tasks. This may be good if John likes football and if Ellen likes ballet, but it just so happens that John is an enthusiastic ballet aficionado and Ellen is a passionate football fan.
I think the whole phrase "but it just so happens that" needs to be considered.
… "but it so happens" means "but the fact of the matter is", "but the reality is", and so the point would still be clear without the "just".
I feel "it just so happens that" subtly emphasizes the convenient pertinence of the fact to the point being made.
This sense of "convenient, fortunate" is very common for this phrase.
"She had lost her purse, but it just so happens that I had enough change in my pocket for the parking meter".
"It just so happens that I was walking past the house when I saw the flames, so I called the brigade".
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