Thanks for your answer which gives a possibility for these kind of situations as well.
I have seen the reverse as well in a new weighing machine where C < ( A + B ) does the sponge material available in it makes the difference ?
I Found that my body weight with shoes is found less than the sum of ( body weight without shoes + weight of shoes ).
example : Weight of body is A and Weight of shoes is B
Human weight wearing shoes is C
I Found that C > ( A + B )...