Mainly because they are different I suppose.
Yesterday DS2 (9yo) committed a misdemeanour which is more usually committed by DS1 (11yo). There is a very standard punishment, issued without fail when DS1 does it (unusual now, as the consistent punishment has been effective)
Anyway, when DS2 was caught, I issued the same punishment calmly and left the room. The effect of doing this to DS1 would have been lots of shouting and telling me what a terrible parent I am etc and my feelings on the matter would be "you deserve your punishment" etc.
DS2 cried! Was heartbroken about the privilege being removed and had a list of "evidence" about why it wasn't fair, Ds1 wasn't treated as harshly etc. It made it so much harder for me to stick to what I'd said. I did, but it was so hard. 