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a2 buffers calculations

i have a question regarding buffers calculations...
the original ka equation (not in buffers) is usually ka= (H+)(Y-) / HY where concen. of (H+) and (Y-) are equal... but when we are using it to work out buffers, since there is a salt, we use the concen. of Y- as the amount released by the salt and concen. of H+ remains as this (based on dissociation of the acid).... this means that for Y- we neglected the moles that were released from the dissociation of the acid (the original conc. before adding any salt).. why did we do this! is it bcz the amount is very tiny so it is neglectable compared with the amount of Y- given from the salt??? sorry for the long question :frown: