I think you have to understand why they build their own. Some do it because they need to drive the price down on certain products or product lines. Some do it because they build so many different types of speakers its easier to build these slight variance in models on the same machines (JBL, Focal). Some do it for performance reasons and the OEM part they need is not available (ATC). Once you understand that, then the next thing to know what is the QC procedures applied. Many manufacturers have a +/- 10dB "pass" windows (although few would admit it). This is not a super high end performance driver, but it could be an acceptable live sound high power driver. Some of them have smaller pass windows, like +/- 6dB or they don't really test them under realistic loads. I would expect almost none of the OEM or in house speaker manufacturers use +/- 2dB QC windows. Few of their customers have a reliable way to measure anything to that small a QC window and wouldn't really know if it was that tight or not. It requires substantial investment in measurement gear and facility to pull that off in manufacturing or incoming QC over and over all day long.
Brad
Brad