Assay over 100% (L-Cysteine Hydrochloride Monohydrate)
There is a simple reason to have the purity greater than 100% for this compound.

If the substance was exposed to a dry environment for several hours, a small amount of the  water of hydration could be lost, causing the calculation to have a higher purity.

Also, I believe that the purity is calculated from the perchlorate titration of the substance in acetic acid (I will need to check the USP monograph). There are many sources for error in the calculations: the weight of the L-Cysteine Hydrochloride Monohydrate, the volume of the perchlorate solution, the error from the standardization of the perchlorate solution, etc.

Under the right conditions, you can easily get 101% for the purity.