Why are NH rd bales different diameters in same field?

I just bought 90 Coastal Bermuda rd bales that were baled by a NH rd baler. The bales were advertised as from same cutting & same field. Why are the bale diameters so noticeably different diameters??  I fully understand how windrow size & ground speed affect bale diameters but I doubt these 2 factors will change that much in same flat field.   I check weighed one GN load of 17 bales and they weighed an average of 991#s

Thanks,Jim

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Why are NH rd bales different diameters in same field?