Well, part of the problem is that the cam lobes/lifter bores aren't really directly oiled at all. They merely get some oil dripping down from the rocker shaft (which does have an oil supply), and then leaking down around the lifters and onto the lobes. I think most of the time, most of the lubrication comes from windage - from the oil droplets flying off the rod bearings as the crank spins.

So in the vein, it makes sense that you'd want to rev it up at first, make sure the oil pressure is up, and the crank is spinning, to make a nice cloud of oil inside the bottom end of the block.

You should also have pre-lubed the lifters, new cams usually come with a little container of grease just for that purpose.

Over the years, the cam lobe/lifter interface has given me the most grief on B18/20 motors, followed by the cam gear itself. The rest of it is all pretty bullet proof, but the lifters are rather highly stressed and poorly lubed.

However good that might sound in theory, though, I have to wonder if the factory ever bothered to do that on the brand spanking new engines rolling out the factory door. Somehow I highly doubt they bothered, and the cam life of cars fresh from the factory wasn't bad.

Another anecdotal theory I've heard is that the original lifters were of higher quality, but then after the B20 went out of production the quality of the Volvo parts counter spares went down significantly.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)