Because the resonance frequency of a SAW device is not simply given by the distance between IDT fingers. There is more going on between two reflectors than that. Fingers have lengths, mass, stiffness, etc. This and more needs to be considered when designing a SAW device.
We can support your SAW project with simulation, design, layout, foundry management, test development, etc. Contact us and we can discuss your resonator, filter, sensor, delay line, etc.!