I knew it'd be related to a unit conversion after the first paragraph. When regulations require units to be metric, but the actual quantity remains unchanged, this is the result.<p>Similarly, in the electronics industry, there are connectors whose pitch (distance between pins) is specified as 2.54mm. Not 2.5, but 2.54, which turns out to be exactly 0.1 inches, or "100 thou" in machinist's parlance. Just to add to the confusion, there are also "pure metric" parts originating in the East with exactly 2.5mm pitch, which seems to fit the imperial ones, but only when the pin count is low enough that the difference is within tolerance.<p>It's worth noting that most if not all of these odd values seem to be metrications of customary units; I can't seem to think of any examples of the opposite.