Suspect results from iostat--FBSD bug?

[Peter Steele]

We use iostat to collect statistics of hard drive activity. We've been seeing some values for the transaction wait column that look suspicious. This is easy to reproduce by just running iostat repeatedly over a short period of time, as I show below. Notice the third from last column. From what I understand, when run in this fashion the values displayed are averaged over the system uptime. I'd expect then for the transaction wait value to not take these sudden dips. Is there an explanation for this?

# for ((i=1; i <=100; i++)); do iostat -dxI ad8|tail +3; sleep 5; done
ad8      10291.0 569044.0 151986.0 10164944.0 4294967295  47.7  93
ad8      10304.0 570070.0 152012.0 10185395.0 4294967295  47.6  93
ad8      10312.0 571047.0 152028.0 10204575.0   85  47.5  93
ad8      10317.0 571729.0 152038.0 10217633.0 4294967295  47.5  93
ad8      10321.0 572363.0 152046.0 10230152.0 4294967295  47.4  93
ad8      10324.0 573863.0 152052.0 10247668.0 4294967295  48.2  93
ad8      10325.0 574438.0 152054.0 10259296.0 4294967295  48.1  93
ad8      10330.0 575150.0 152078.0 10273362.0 4294967295  48.1  93
ad8      10332.0 575693.0 152082.0 10283567.0 4294967295  48.0  93
ad8      10334.0 576478.0 152086.0 10298971.0 4294967295  48.0  93
ad8      10337.0 577363.0 152092.0 10316766.0 4294967295  47.9  93
ad8      10340.0 578501.0 152098.0 10328958.0 4294967295  48.4  93
ad8      10343.0 579682.0 152104.0 10352494.0 4294967295  48.3  93
ad8      10349.0 580806.0 152116.0 10375251.0 4294967295  48.2  93
ad8      10356.0 581462.0 152200.0 10387402.0    0  48.1  93
ad8      10362.0 582392.0 152212.0 10405738.0 4294967295  48.1  93
ad8      10367.0 583326.0 152222.0 10424055.0 4294967295  48.0  93
ad8      10367.0 584412.0 152222.0 10432981.0 4294967295  48.7  93
ad8      10376.0 585764.0 152240.0 10460020.0 4294967295  48.6  93
ad8      10377.0 586649.0 152242.0 10477525.0 4294967295  48.5  93
ad8      10382.0 587266.0 152252.0 10488950.0 4294967295  48.5  93
ad8      10392.0 587977.0 152272.0 10503115.0 4294967295  48.4  93
ad8      10402.0 588713.0 152292.0 10518177.0 4294967295  48.4  93
ad8      10403.0 589831.0 152294.0 10527746.0   16  48.8  93
ad8      10419.0 590811.0 152326.0 10547455.0 4294967295  48.7  93