What does a decrease by a negative amount mean?

Recently I caused some share movement by amalgamating several holdings into one. As a result of my action, I received notification from the registry describing the movement.

But I am puzzled by this: under the column marked 'Decrease', it has a negative amount. To my mathematical mind, I would have thought a decrease (negative of increase) of a negative (double negative) would produce a positive number. Like calling a C++ function like this:

int Dec( int &var, int amount )
{
     return var -= (amount);
}

So if you have this:
int x = 100;
int y = Dec( x, -10 );
assert( x+10, y ); // as any logic will tell you.

If the registry intends on using negative amount to indicate reductions, it should display that figure in the 'Increase' column and the sum would come out right. Perhaps the company has not gotten around to amalgamate the 'Increase' and 'Decrease' columns into one 'Movement' column.