Most voltage controlled synthesizers follow a system where an increase or decrease of 1 volt corresponded to change in pitch of one octave. For example, if 1 volt corresponded to middle C (C3), playing one octave lower would require a control of 0v, and playing one octave higher would require 2v. This requires not-straightforward exponential voltage convertors, which can be inaccurate or drift with changes in temperature.
Therefore, in the earlier days some manufacturers (such as Korg and Yamaha) followed an alterative standard where a change of 1 volt corresponded to set number of cycles per second – for example, 1 volt may correspond to middle C (~261.63 Hz); one octave lower would require 0.5v, and one octave higher would require 2v. This system is arguably cheaper and more stable (which is why it is reappearing inside some self-contained analog synths), if not as straightforward when it comes to voltage control in a freely patchable modular system. It is possible to buy conversion boxes that translate between the two.