Well the WU difference could be it too. For example, unit A was at 5000 ppd. new core gets it to 5300 ppd. Cool. Unit B was at 4800 ppd. Now it is at 5000 ppd. Cool again. However, since you moved from unit A to unit B, it appears that you didn't get a boost, because you went from 5000 to 5000, when you really did get a boost.. I'd check it next unit and see how things look after about 5%-10%.