Matlab c2d(1/s) is different than z transform tables
sys = c2d (sys, tsam, method)


If you do not include the method it will use zero-order hold

c2d(G,1,'zoh')


resulting in 
$$ \frac{1}{z-1} $$

If you pick a different conversion method it gives you different results, but notice that the poles stability will be "conserved"

c2d(G,1,'tustin')


results in 
$$ \frac{0.5z+0.5}{z-1} $$

c2d(G,1,'impulse')


results in 
$$ \frac{z}{z-1} $$

c2d(G,1,'matched')


results in 
$$ \frac{1.052}{z-1} $$