33 mm plate in tanks

After Design , during Fibraction .. if the contractor dosen't find the design thickness in Market, the contractor can avail plates slightly above the design thickness to avoid excess weight.

This is assuming you are talking about the first shell course. Typically, I trust the tank/vessel fabricator. The additional thickness could be a result of several factors. Minimal wall thickness required by design. Corrosion allowance required. Nominal loss during polishing activities (If you are at minimum after polishing and you have repair work in the future, you will have to "over-lay" so as not to polish beyond minimal wall thickness.) Loss during head forming at the knuckle area. (I realize you are not ref heads, but if they did a bulk matl buy-out, they would by like matl) Market availability is another factor. Also, maybe they had the material in stock from a previously canceled project. Weld joint efficiency could be another reason. At times a fabricator will increase material thickness to affect overall weld joint efficiency and reduce potential NDT requirements/costs in the field. My concern would be, depending on amount of material, the additional weight on foundation, legs, skirt etc. but even so, we are not talking significant amounts.