I believe it's by design. Differing indicator frequencies make the signals more noticeable.

Back in the old days, turn signals were actuated by bimetallic thermal relays - a simple and highly reliable device. They were constructed with a range of frequencies by manufacturers and sold mixed together, so cars might receive the same part number but with an essentially random on/off period. This part would also mechanically create the familiar clicking sound of a turn signal actuating.

These days indicators are controlled electronically and I assume the randomness is programmed in.