Apple will only specify what they have tested themselves, and in some models of about that time the memory technology advanced just after a model had been discontinued. For glaring example, all models of the Beige G3 accommodate 3 * 256 MB DIMMs of the right type, but Apple only certified the last model as accepting up to 128 MB DIMMs (earlier models had been completely discontinued by the time the denser RAMs became available, so Apple never tested them and never revised its documentation).
It is important to buy from a vendor that actually tests RAM in Macs, not just maintains a database of what sales "stick" and what sales are returned.
