does the balanced field length ALWAYS have to be equal ?
By definition, the answer must be "yes", providing we modify the question to specify ASDR=TODR. Note that the concept of BFL doesn't relate to ASDA and TODA as that wouldn't be terribly meaningful.
I think what they probably are getting at is more along the lines of "do you always need to have a BFL calculation ?" to which the answer is, subject to the AFM's scheduling non-BFL data, "no".
By way of explanation, generally BFL will give you the quickest and, possibly, best weight for a typical runway. However, if there is a noticeable difference in ASDA and TODA then, depending on the ASD versus TOD characteristics of the particular aircraft, unbalancing the takeoff may provide a weight better than for a BFL calculation.