They are standardized for a given mix. A mix design that is based on a trial badge is submitted to the SEOR prior to pouring anything. The mix design shows the ratios ingredients (cementitious materials, find and coarse aggregates, water, air, admixtures). But Concrete is still a non-homogeneous material with lots of variations. Take for instance aggregates, if it rained the last two weeks, the moisture content will be higher but it may only be a layer on that pile. Same goes for gradation (particle size of the rock). Sometimes you get a batch with smaller rock. There are a 100 things that can go wrong to get bad mud.

But yeah, there are concrete plants that cut corners and try to save on cement (the most expensive part of the mix), which depending on the project may bite them in the ass when they have to pay to fixing it.