When you began writing the description to the problem you actually seemed to agree with jmossel as you wrote

"(55, meaning -4.2 compared to the average) ... (95, 35.8 more than the average and so on)"

where the signs of the differences are as expected. You then later contradict yourself by writing

"The resulting list/array of differences starting from [55, 95, 62, 36, 48] is thus [4.2, -35.8, -2.8, 23.2, 11.2]."

A tip to improve the description would be to change the first quoted sentence to make it consistent with the second (I would say change the second to be consistent with the first, but that would make all existing solutions to the problem incorrect).

I also don't see how simplicity, beginner-friendliness or empathy with the beginner caused you to make this choice of sign (and to be inconsistent about it in the description).