These terms do not appear to be used consistently in the literature
In this classification, it seems they are possibly using prohormones specifically to indicate protein/peptide precursors to peptide hormones (similar to the term "proenzyme"), and prehormone for non-peptide hormone precursors. In this terminology, prohormones will always be a longer peptide chain than the hormone, and the hormone is formed by proteolysis. In that case, preprohormones are just peptides that are cut to make a prohormone, so they are longer chains than the prohormones.
It's also possible they are intending to separate based on where the conversion takes place, and a prohormone is modified to the final form in the source tissue, whereas a prehormone is released and then modified in target tissue. An example from an old paper, Baird et al 1963:
Emmens  has defined a prohormone as a substance which exerts its
biological effect by peripheral conversion to a more active compound.
Prehormones may be defined as substances normally present in the
body and usually secreted by endocrine glands, which have little or no biological potency themselves but are converted peripherally to more
I don't think these delineations are particularly consistent from author to author... T4 is called a prohormone, too, sometimes, and at least in Google Scholar appears with that word much more often than "prehormone".
Some urge that the delineation is on whether the modification occurs locally or elsewhere; for example this paper argues T4 should not be considered a prohormone because a prohormone is something produced and converted to the final hormone form locally, in the same gland, whereas prehormones are modified to their final form in target tissues.
Yet another case, this paper refers to a peptide hormone where they call the longest chain "prehormone" which is then shortened to a "prohormone" which is finally shortened to the final product.
For your course, I'd focus on whatever distinction your textbook/instructor wants to make, but for the rest of life (and this is general advice for all terms in biology) I'd advise:
Make sure you know the basis of what you're being taught, not just the term. Most terminology in biology is just labels, you cannot reason backwards from the term to learn about the world, the term is just a shorthand to cluster some concepts together to organize communication and knowledge.
Recognize that terminology is not consistent and you always need to clarify what is specifically meant. Authors of papers and textbooks are not necessarily consistent, as they may have different focuses and values. See also https://en.wikipedia.org/wiki/Lumpers_and_splitters for one cause of this, and https://xkcd.com/927/ for one result when people try to "fix" things to be more standard.