Please forgive this silly question, I'm fairly new to statistics.
Consider this R code:
a = c(1,2,3,4,3,2,3,4,5,5,6,5,4,3,4,5,6,7,8,7,6,6,5,6,7,10,9) b = c(10,9,7,6,5,6,7,8,4,6,6,5,4,5,6,5,4,5,6,7,5,4,4,5,4,3,2) mean((a - mean(a))*(b-mean(b)))  -2.42524 cov(a,b)  -2.518519
why are these two values different? Are the mean and expected values not the same thing?