in some diseases the loss of hco3- is balanced by anions of the acid eg lactate so the positive(cations) ions are increasing and the negative (anions ) are decreasing
the equation is( na+) -(chloride ion +hco3-) so as i said above if u think that statement accoding to this equation there is an increase in anion gap
but suppose say in diarrhea loss of hco3- is counterbalance by cl- so they balance each other so there wont be any change in the anion gap
hope u got it