Calibrated and variable thickness paving stone will both look the same on the surface - the difference is what happens on the underneath of the stone. 

With calibrated stone, it is cut to an even thickness, often with a corrugated or partly corrugated back to the stone. The benefit to calibrated paving is that it is often considered easier to lay as it can be set onto a predominatly level wet mortar bed and very little adjustment will need to be made between the slabs as the overall surface level is usually fairly even. With calibrated stone, it is often cut to a depth of 18mm, 22mm or 24mm for example, however it is important to note that in accordance with British Standards, there may be a small tolerance on the dimensions.

With variable thickness stone, the depth will range between two measurements, for example 15-22mm or 25-35mm (plus permitted British Standards tolerances) and this means that when laying the paving it is necessary to adjust the level of each paving slab within the mortar bed as you go using a spirit level. 

It is important to note that there is always the necessity to work with levels when laying all paving to ensure the overall run of the patio slopes away from the house to avoid water ingress. Although calibrated paving is considered a quicker laying solution by some, variable paving is usually better value and does not take significantly longer to lay, so is often a good, cost effective option.