Whether a bike has stem shifters, centerpull front caliper or a stem shorter than 8cm or so can affect where the optimum cable routing position will be.

I've surprised myself a few times, when I ended up routing cables in front of the handlebar for one or more of the above reasons.

None of the bikes I ride has a stem shorter than 8cm, so I route the cables behind the bars even when stem shifters are used.

I don't think that the factories were always very particular about where the cables on boom bikes were routed, as evidenced by some of the catalog photos, and I think that it's possible that simple things like packaging/shipping concerns could have swayed the decision on how cables would be routed.