Here's why:

* Cyclic Alkanes: Cyclic alkanes have a ring structure, which means they have fewer hydrogen atoms compared to their acyclic counterparts. The general formula for a cyclic alkane is CnH2n, but the ring structure results in a loss of two hydrogen atoms, leading to CnH2n-2. Since they are "deficient" by two hydrogens compared to alkanes, they fit the CnH2n-6 formula when you consider that they are also missing four hydrogens compared to alkenes (CnH2n).

* Alkynes: Alkynes contain a triple bond, which involves the loss of four hydrogen atoms compared to the corresponding alkane. The general formula for an alkyne is CnH2n-2. Since they are "deficient" by two hydrogens compared to alkenes, they fit the CnH2n-6 formula.

Examples:

* Cyclic Alkanes: Cyclohexane (C6H10)

* Alkynes: Hex-1-yne (C6H10)

Important Note: The CnH2n-6 rule is a simplified way to represent these compound types. There are exceptions, such as cyclic alkanes with multiple rings, which may have different formulas.