Those are normal variations.

A quarter inch? The extra quarter inch is there they just put connected one side too low

The picture of your tape-holding is a little deceptive; it's the angle. Squeeze two small straightedges between the bottom and the top, hold the tape between them, and take the picture from dead straight in front, please. I'll bet you don't have 1/8".

I’ll post another picture but it’s very obvious to the naked eye. The price difference was actually $20 for the prefab vs whole slab. Problem is the whole slab was on back order for months and we needed the project done.

More pictures

Prefabricated counter means Jj took a risk in order to save money. Prefab vs buying the slab.

http://lgsgranite.com/tips/whats-difference-prefabricated-slab-countertops/

Jj, your installer (is not the fabricator), did the best he/she could with the prefab quartz you chose. You have to balance saving $20.00 per linear foot with the outcome.

In other words, "you got what you paid for”. Not bad!

Do over. Not because of the variation in thickness but because you don't have enough overhang past the cabinet fronts. Looks like the overhang is even with the fronts or that the fronts are a bit proud of the counter edge.

It is a common practice to fit adjacent pieces together in the shop and grind the underside of the thicker piece to eliminate the lippage at the joint. However, not all shops do it as it's an additional step in the shop instead of simply placing the finished parts on a frame for install as they come off the CNC or polishing line. This is necessary because the thickness of slabs does vary even within a specific slab. If you paid a low ball price you got what you paid for.

I agree with ci_lantro. The overhang is too small. Having said that, even prefab stone counters are expensive so no need to hate on the OP because he/she expects a level of quality.

There is no level of “quality” to expect with prefab. It’s as cheap as it gets, and that never equals quality. They come in one depth only. No one cut them too shallow. That’s just how they are.

I just did a quick search of prefab quartz counters and they are all 26" deep. The OP was definintly misled by whoever sold them those counters.