A nonsense suppressor mutation is a kind of mutation that makes the tRNA base-pair with even a stop codon, by misreading the codon. For example, UAG (stop) can be read as UCG (serine) in nonsense mutant.
As a result, the nonsense suppressor can read the whole mRNA strand and synthesize a complete polypeptide. But, the polypeptide produced will not be exactly as the original polypeptide.
The nonsense suppressor has a chance of coding wrong codons, that is, one amino acid for another. For example, it may recognize UGG instead of UAG and place tryptophan instead of stop codon.
As a result, the polypeptide produced will not be active and the cell will not be exactly wild type. The nonsense suppressor although can correct a non-growing mutant, it cannot produce a cell, which exactly resembles original cell.
In addition, the translation of the mutant gene will not be as efficient as the original gene and may cause premature termination. This leads to less product, producing a state, which is “not exactly wild type.”