Tank Top Plans

Quick question more related, I think, to the plans themselves than to the actual ship - what is the reason/meaning behind the uneven external hull lines shown in the forward area of the plans for the tank top?
 
The lines run the length of the tank top, their explanation is written between BR2 and BR3. The narrowing of the ship and the resulting rounding under the hull makes it very hard to draw a single line in the plan view for where the side of the ship is.

Bill
 
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