You're kidding, right?

I FTFY:
Two plugs of the same design and construction, that meet the same specifications and quality criteria, and different brand, may be the same plug.

Products coming off a production line are routinely tested. There are always production tolerances. Routinely, those that meet different test specifications are distributed through different channels, often at different prices. It happens with everything from oil filters to spark plugs to computer chips that get clocked faster.

Working for an engine manufacturer and as a distributor of their engine parts, we often ran into customers purchasing cheaper filters from the same supplier our engine manufacturer used, and claiming they were the same. We routinely cut them open to show them the filter construction differences, and tested the filter performance with portable particle counters, to demonstrate the different filter media used by the filter manufacturer, on a single production line.

The cheaper part may work just fine. Or, there may have been a reason for the manufacturer's original specifications, in which case it may not work fine or last as long. I am not saying they will all fail, but it isn't correct to say that they are always the same, just badge-engineered.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue