[R] Regression Tree Question

[t.shen at stat.ubc.ca]

Dear all,
I have a question about the regression tree function in R.

After applying  the tree() function for my data, I found out the
"n" value in the output (the (weighted) number of cases reaching that node) is
always less than my total sample size.

My guess is becasue some of the cases are not useful to predict the  
node. I am not very confidence with my guess and I do expect to know  
some "formal" explaination.  Thank you very much!

Cheers,
Jessie