Sorry Bill, but when specifications are included in a "contract document", that tolerance cannot be arbitrarily changed. That is why there are contracts and the specifier is responsible for that information. As unpretentious as making a 3-1/2" slab instead of a 4" slab might seem, it is still not what was asked of the contractor, and he is in error and liable for that change, if he indeed make an arbitrary change. Depending upon the situation, the elimination of the correct depth of concrete could very well create economic hardship for the home owner.
It is these "field standards" developed by well meaning, and not so well meaning, contractors that has created a very lucrative field. The field of building inspectors.
In this case, I strongly underline the need for documentation as I previously submitted.