It's your memory. You are mixing it up with the Trail's responsibilities. Although, the trail does potentially have a better view of that top space that the lead is supposed to cover. I suspect it is a matter of keeping the officials area of coverage from being too scattered.
From the 1997-1999 Official's Manual book:
242. Trail official ... will:
e. Watch the top three spaces on the opposite side for lane violations...
243. ..the Lead official...will:
a. Observe the opposite side of the lane plus the first space on the near lane...
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