Why am I getting a weird measurement for a driveshaft?

I measured from the flat/farthest forward point on the yoke to the seal...

Just re-read. I think most length quotes are center of front U joint to center of rear U joint. that would account for some of your length difference. I don't have the slip yoke seal but you can see there is at least 2-3 inches from center of u joint to where the seal would be
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This is another members photo with the seal and the same applies
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