That trapezoid thing is too real. I swear my tape measure grows or shrinks an eighth every time I put it down. Did a simple shed roof last month, and by the fourth rafter I had four different lengths. The story pole doesn't lie, but my brain sure tries to.
Honestly, the worst is when you're marking a bunch of blocks or something and you just KNOW your pencil is getting duller with each line. That little fudge factor adds up so fast. @the_ben is right about the tape playing tricks, but my own lazy marking is a bigger enemy. I once laid out a whole deck post pattern freehand, convinced I could keep it square by eye. Let's just say it had more character than the plans called for. The story pole is basically admitting you don't trust future-you, and future-you is NEVER trustworthy.