There is a good possibility to explain it in a following fashion:
If a civilization exists on a specific planet that is sufficiently developed to peak Borg interest, they could probably swoop up the cities and just convert them to energy for example and 'consume' them while leaving the rest of the planet.
If a planet on which a sufficiently advanced civilization exists and has properties of interest to the Borg, they could assimilate the entire planet.
They would likely convert the entire planet to a suitable environment for them and use it as efficiently as possible using their nano-probes (effectively speaking, terraform the planet to cater the Borg).
They could probably do this with planets with no civilization on it.
Technological and biological interests may not be the only significant aspects to the Borg ... if it's a suitable environment the Borg could use for themselves in one way or the other, then they will assimilate it and adapt it for whatever it is they want to use it for.
Should the Borg decide to assimilate a planet that contains both a highly and less advanced civilization ... they will either assimilate everything and everyone, or just assimilate the majority and exterminate the ones they don't perceive as worthy (although there were Trek episodes with 2 races evolving on one planet with the less advanced one being more suitable for survival and having sufficiently developed cerebral abilities that can be used for the entire collective as processing units ... they can easily convert a villager with no technology or anything into a skilled person via assimilation ... have access to the collective knowledge/understanding of thousands of species ... and after potential 'liberation' from the hive, this individual would be one of the smartest ones in the Trek galaxy likely).
Any number of extra drones could be beneficial to the collective.
If they assimilate existing planetary culture, they newly converted drones there can finish up the assimilation process of the planet and use it for Collective's purposes.