Mass percent composition
The IUPAC Compendium of Chemical Terminology defines the mass fraction $w$ as the


  mass of a constituent divided by the total mass of all constituents in the mixture


$$w_i = \frac{m_i}{\sum\limits_{i=1}^{N}m_i} \tag{1}$$

This means that $w_i$ can adopt values between 0 ( = 0 %) when the compound in question is not contained in the sample), and 1 ( = 100 %) when the sample only consists of the compound. 

If your sample isn't totally dry, the mass of water adds to the total mass of the sample. 

As a result, the denominator in $(1)$ will become larger and the mass fraction $w_i$ for any component (except the water) will decrease.