They do and many factors allow this to be the case, component choice, component
brand/spec/ tolerances, circuit topology, curves, proportional Q or not, eq circuit
design, layout and type, physical construction, pots vs switches in stereo eq's, power
supplies, transformer or electronic balancing of I/O, PCB layout, solid state vs valve,
solid state vs monolithic opamp based, analogue or digital, band bypass switches or not,
input and output impedance of unit itself and to that which it is connected.
All different, some subtle, some virtually inaudible and some obvious, any combination
of the above may reside in any given product or design, none sound the same although do a