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Salam waqac nominal shear strength is the sum of the two 1.shear strength by concrete,Vc and 2.shear strength by steel reinforcement, Vs. the expression for the Vc you mentioned is used for manual calculations and is a bit conservative and ACI put a limit on it for the sqrt(fc') should not be greater than 100.also this expression is based on the test results for concrete fc' 3000-5000psi,when we increase the fc' shear strength of concrete decreases further than 6000psi.at first there is shear is only resisted by concrete and there is no steel reinforcemnt is required at all but ACI requires a minimum shear reinforcement.so at first only small shear reinforcement adds greater than to Vc which might be greater than the expression you mentioned actullay.there is limit of Vc must not be greater than 3.5 sqrt(fc')bd,it can actually be greater than this but ACI have put the limitation.so this could be the reason for the difference in experimental and theoretical results. if you have mentioned the compressive strength of concrete you used in the tests and the the location of beam X-section where you have observed the shear crackes one can comment more easily.