Is it normal to see variation in manufactured quartz countertop sections?
Every slab is bound to have variations. One slab could even have variation from one end to the other. 

If your top supplier got two slabs that were cut off a block right next to each other it should be common to create joints that butt the two ends of the pieces that came from the same end of the block. On the other hand if making two pieces from the same slab then the butt joint should be made near the end to end cut of the larger piece. 

However that said there are likely to be many reasons as to what should be have been done was not done. Could be less skilled cutters/installers. Could be one of minimize cutoff waste to reduce cost. Plus others...