Quality standards are not reduced over time, but the number of units getting a major inspection off the line can decrease. In order to maintain efficient throughput in manufacturing, not all aspects of the assembly are inspected at 100%. Rather, random sampling is done at various points on the line to try and establish a confidence level in the units being built. Normally, during the first builds, Acceptable Quality Level (AQL) values tend to be stringent so that inspection sample sizes may be larger, and this is done so that the confidence level is greater for an acceptable number of units. If the inspections on those sample sizes falls within the acceptable range with an acceptable deviation from the nominal, inspection sample sizes may get smaller over time due to a reduced AQL on a well-established procedure.
I'm sure there is a standard final quality assurance procedure that is done 100% as the final step, but it is probably not as detailed as a complete inspection based on the AQL sampling of a manufacturing lot. The same also applies to the components in a lens (e.g. optic components, lens housing, etc). So, it's completely believable that there is some unit-to-unit variation since not all components are inspected at a 100% level (a lot of validation and six-sigma work is probably done in parallel to nail down the process for the manufacturing of the components too).