needing explanation: GPU clock versus VGA core clock

[vois2]

Hi everyone. Thanks to helpful advise from folks at Subsim.com I've removed my Ati RADEON x300se and replaced it with a BFG 6600 GT OC. Now, that BFG card comes overclocked out of the box, which leads me to this question:

When I check graphics clock speeds in Everest Home and PCMark05, I get different numbers. Can someone explain this to me please? Specifically, I don't understand the difference in the two reports below, regarding memory clock.

Here are the specifics:
P4 630 HT , 1 gig RAM , 6600 GT overlocked to 525 MHz

Everest Home readings:
Graphics Processor Properties GPU Clock 526 MHz
Memory Clock 526 MHz
RAMDAC Clock 400 MHz

Everest Home
Memory Bus Properties Real Clock 526 MHz
Effective Clock 1053 MHz

Yet PCMark05 reports these:
VGA Memory Clock 1.1 GHz
VGA Core Clock 299.3 MHz

Thanks in advance!