Well, even if we assume that the factory has perfect environmental controls and their plastic mixes are completely homogenized and consistent, depending on the size of the operation there can be a few sister molds or dozens or hundreds. Minor variations from mold to mold could easily explain why one person gets tabs that lock firmly and someone else gets tabs that don't work at all.
It gets worse as the molds get polished and repolished and tolerances continue to decline. Just look at fit and finish of 1st run toys versus later reissues. You'll see stuff like parts not matching up as well, hinges and other high tolerance areas being loose or not fitting properly, excess flashing in some cases or lines and detailing just not being as clean and sharp.