Warping used more yarn than calculated?

I have a Clover Bloom Loom which uses a hybrid warping style (the loom doubles as a warping board and you thread the “heddle” as you measure the strings. I set up all my warp only to find that I just barely had enough of the yellow. According the math, I was supposed to have ~50 yards of it and I was left with ~6 yards. That’s a huge amount to vanish! It used ~13% more yarn than expected!

I then made up a guide string to measure, doing my best to use the same tension, and even using the farthest slot (which should have the greatest length bc of diagonals), it was 395.5 cm. Which is actually 0.5 cm less than in my math.

I’ve calculated and recalculated and everything keeps turning up the same numbers, keeps saying that I should have more yellow left. The only thing I can figure is that the skein was underweight, but having it be 10% underweight is not very plausible.

Is there anything else I’m missing? Any other factor to explain why the yarn is missing? I’m so scared to take it off the pegs and cut the yarn, if there’s something obvious I’m missing. And that kind of affects all future calculations, knowing that the warp could eat much more yarn than expected.

=> The Math: There’s 81 yellow ends and 36 purple ends. They should all be 400 cm long. The yarn is 366 meters per 100 grams.

400*81/100=324, so I should have used 324 meters.

6 yards =5.486 meters. 366-5.5=360.5 meters is how much I actually used (unless I was starting with less than 366 meters).

(sorry for jumping between units, my phone isn’t letting me go back to edit the yards into meters/g. I use imperial, but the loom and its instruction manual use metric.)