I remember back in the 1980’s that Sendra, Pac West Arms (PWA), and Olympic Arms offered “matched sets”, where the factory would custom-fit an upper and lower receiver together. Those early AR manufacturers also produced receivers with a “tighter tolerance” than Colt, which limited interchangeability, but made for really tight fitting parts. Some people confused this “tighter tolerance” for the receivers being “out of spec.”, but that probably wasn’t the case.
The usual formula to create the tight receiver fit was to machine the lower receiver with a taller than normal upper deck height. This, in essence unnecessarily made the upper deck of the lower receiver into a “critical dimension”, subject to potential, variable interference with an upper receiver. On a Colt lower receiver, only the spacing between the take down and pivot pins forms the critical dimension for upper/lower receiver fitment. Colt just kept the upper deck of the lower away from the lower deck of the upper, if ya know what I mean.
Unfortunately, over the years, many of these early aftermarket AR lowers have been modified to have a mil-spec deck height, to allow for the use of unmodified upper receivers. I guess that some people prefer “standard” over “custom”, but that is their choice.