The interest goes beyond being a hobby as the nerd using nerd cover will either avoid or down play other subjects that might reveal their innate personality. Nerd cover can be used to redirect a conversation from traditionally nerdy subjects to being used to dilute obvious nerdiness in an individual's interests.
"Maybe, but did you know he keeps goats and chickens?"
"That's just his nerd cover, don't get him started on the latest zombie film."
"Wow, Warren's fantasy football team is unbeatable!"
"Yeah, football is his nerd cover. He's a mathematician so the numbers come naturally to him.