Hi Armin,
the capacitive loading of the wire coating and the fiberglass tubes around the wire are the reason for that short length. It’s a factor of appx. 0.92 compared to common YAGI designs.
I determined the element length by simulation using CST, a professional EM field solver for antenna design. NEC based simulation tools can’t deal with dielectric materials.
There’s a whole chapter in the pdf - “Discussion on electrical design”
Maybe that answers some more questions.
73
Martin - DF6MH