How can a Zn-Cu galvanic cell have a voltage of 1v in stead of 1.1v
You can get a smaller potential if you alter the concentration of one or both ion solutions away from ideal 1M concentration. This behavior is described by the Nernst equation:

$$E_{cell}=E_{cell}^{o} - \frac{RT}{zF}lnQ$$

where $E_{cell}$ is the actual potential in the cell, $E_{cell}^o$ is the potential of the cell under ideal conditions, R is the ideal gas constant, T is the temperature, F is Faraday's constant, z is the number of electrons in the reduction oxidation reaction and Q is the reaction quotient. By altering Q to be greater than 1, the actual voltage of the cell dips below the ideal value.