Electronic Devices - Special-Purpose Diodes - Discussion

One of the drawbacks of using just a single varactor diode in certain applications, such as rf tuning, is that if the diode is forward-biased by the RF signal during part of the ac cycle, its reverse leakage will increase momentarily. Also, a type of distortion called harmonic distortion is produced if the varactor is alternately biased positively and negatively. To avoid harmonic distortion, you will often see two varactor diodes back to back, with the reverse dc voltage applied to both devices simultaneously. The two tuning diodes will be driven alternately into high and low capacitance, and the net capacitance will remain constant and is unaffected by the RF signal amplitude.

Why not tuning range? Please explain.

Why not a wider tuning range? Explain.