Many studies have been directed to probe ring oscillator PUF's feasibility in the security field, but most of them suffer from the lack of global approach as they analyze the system isolated, giving an uncompleted theory about their behavior. This paper presents how adjacent hardware elements may affect PUF response, modifying their statistical characteristics and even masking the randomness of manufacturing process. This is a factor that should be taken into account when modeling the behavior of the ring oscillators in the PUF. Experimental results from Xilinx Spartan 3 FPGAs illustrate these issues.