I agree with you!
The "multifid back lobes" are NOT an identifying characteristic of this species - but occasionally they occur. I'm sure one can equate the "back or posterior lobes" to the congenital birth defects that occur in animals. They are aberrant or incomplete divisions that are not genetically programed but form atypically during rapid cell division due to a circumstantial and probably temporary external influence.
I just counted 24 leaves on my four plants - only 4 leaves showed incomplete separation of the pinnae nearest to the petiole; in those four leaves, the pinnae in question were cleft all the way to their bases and each lobe had a separate primary vein arising from the central leaf vein.
The TGOA line-drawing OF A SINGLE LEAF with four lobes within the one pinna and three in the opposing pinna certainly shows a very, very atypical leaf! I have grown my clone of P. tortum for nearly twenty years and have never such multiple (i.e., 3 to 4) lobes.
One of the "spider philodendron's" characteristics that first stood out for me was - that the basic leaf outline of P. tortum is essentially oval - the longest pinnae and the widest part of any leaf is usually at mid-blade. Most other Philodendron are more triangular or hastate in outline and usually widest near to the petiole.