That is not so odd, when you think about the production process. There are lots of chances along the way that could cause this sort of variation. When a company manufacturers cranksets, they do so in as a large production run as possible, to minimize the time cost involved with multiple tooling setups and teardowns. Consequently, both arms are not necessarily produced concurrently. The left arms could sit waiting in large bins while the right is manufactured and/or goes off to the assembly department to have the chainrings attached. Both could sit for a while longer awaiting manufacture of the bottom brackets. Eventually, they will be pulled out the bins, mated and put in a crankset box, but that would only be if intended for retail sale or by a reletively small volume manufacturer.

If the crankset is intended for a large volume bicycle manufacturer, they would typically be shipped in separate, large cartons for the left arm, right arm and bottom bracket. Individual cartons for mated cranksets are an extra cost, as is the time to pack the parts in a separate box. At the bicycle manufacturer's end, the time cost is doubled by having to unpack the mated crankset and separate the parts, as the person who assembles the bottom bracket may not install the crankarms and there may be even different people for the left and right crankarms. It's most efficient, and therefore cost effective, to keep the parts separate from manufacturing, straight through to the assembly operation.

The bottom line is that both crankarms are not necessarily produced at the same time and immediately paired. There are lots of opportunities for one of the parts to experience delays before it gets paired with its mate and assembled onto a frame.