If a picture of a tank confuses newcomers... maybe we don't want 'em designing vessels?
Though I'll tend to agree, a shot of a vessel might have been more appropriate.
As for solid engineering justification - well, we could call it "round off error" or "within an order of magnitude" or something. After all, consider some alternatives: The cover could have had a photo of an airplane (still, generally round, long, and pressurized), a cruise ship (a.k.a. a "vessel"), a pretty scene of colorful fall leaves, perhaps a portrait of some global leader, children playing soccer, folks enjoying the sun at a beach...
But... I gotta wonder why it took so long for roca to notice?
jt