Thread Subject: int in R2008b; additivity of integration on intervals

I am trying to prove the additivity of integration on intervals in R2008b:

>> syms a b c

>> y = sym ('f(x)')
 
y =
 
f(x)

>> null = simple (int (y, a, b) - int (y, a, c) - int (y, c, b))

null =
 
int(f(x), x = a..b) - int(f(x), x = a..c) - int(f(x), x = c..b)

>> null = simple (int (y, a, c) + int (y, c, b) - int (y, a, b))
 
null =
 
0

I do not understand why the second but not the first expression is simplified.