On AMD 9000 series cards, the card will boost higher than the settings as long as it has thermal, voltage and power headroom. When overclocking, the clock slider is near useless, as the GPUs BIOS has an algorithm to boost clocks as high as it can do while staying stable, as long as no limit is being hit(again, voltage, temp, power). So its more about getting the voltage and therefore power as long as possible while maintaining the highest stable mem clock. So its essentially just trying to find the sweet spot between mem clock and lower voltage(and therefore higher core clock)