According to the
New York Roadrunners the Central Park loop is exactly 6.0273 miles.. but that's probably on the inside lane. I use a Polar CS 100 which usually gives me 6.1 or 6.2 and while I do get some variance the only time I've come up that short in terms of mileage was when my battery was going dead. So I'd guess it's either your battery or it's set for the wrong wheel size. I did have some interference when I used the heart rate monitor but never in Central Park.
And don't worry about getting buzzed in the park. Some of the fastest riders in the world like to do a few laps there.
