A difference between aciclovir and penciclovir is that the active triphosphate form of penciclovir persists within the cell for a much longer time than the activated form of aciclovir, so the concentration within the cell of penciclovir will be higher given equivalent cellular doses.
The longer duration of penciclovir's active form inside cells compared to aciclovir's leads to a higher concentration of penciclovir within the cell at equivalent doses.