The correspondence between BRST-BFV, Dirac and projection operator approaches to quantize constrained systems is analyzed. It is shown that the component of the BFV wave function with maximal number of ghosts and antighosts in the Schrodinger representation may be viewed as a wave function in the projection operator approach. It is shown by using the relationship between different quantization techniques that the Marnelius inner product for BRST-BFV systems should be in general modified in order to take into account the topology of the group; the Giulini-Marolf group averaging prescription for the inner product is obtained from the BRST-BFV method. The relationship between observables in different approaches is also found.