CSP. Molten salts- Simulation with imported heliostat field

Hello,
 
I’m trying to run a simulation of a heliostat field importing coordinates from an external file using a fixed solar tower. Once uploaded the heliostats field file, when simulating the performance, I keep both options: ‘Generation heliostat layout using tower dimensions’ and ‘Optimize heliostat layout and tower dimensions’ non activated. Number of heliostats and tower data remains the same before and after simulation.
 
The issue is the following: depending on what the solar multiple value is defined in System Design screen, simulation reaches different values and annual production decreases/increases. As the solar filed is already fixed, which is the reason of getting different values? 

Thanks!