Can anyone throw some light onto the class system that SD card manufacturers use?

After spending a long time with just a couple of 4GB cards (one class 2 and one class 4), I recently bought some higher capacity cards. The first was an 8GB Kingston which was a class 4 card and the other one was a 32GB class 10 card made by Storage Options. Much to my surprise, the new class 4 and class 10 cards were actually 'slower' than the class 4 I already had.

The cards are as follows:

4GB Sandisk Class 2
4GB Sandisk Ultra II Class 4 (This card actually has 15MB/s written on it but states the card is still a Class 4)
8GB Kingston Class 4
32GB Storage Options Class 10

I use a freebie app by AJA at work to test the throughput of various drives, so I decided to use it on these cards to see what sort of benchmarks I can achieve with them. Each of the cards was formatted, then a series of large files were written to and read off the cards via an ExpressCard SDHC reader which actually appears a USB2 device on my i7 MBP. I thought it might be interesting to share my experience with you guys.

It appears Sandisk are the most honest, their cards seem to be over-specced. The class 2 card is actually delivering class 6 performance on write speeds and class 10 on reads. The Ultra II however, blows everything else out of the water with write AND read speeds exceeding class 10. Why this card is described as a class 4 is beyond me.

The Kingston and Storage Options cards however, are delivering the bare minimum required of their class rating for writing, but have read speeds that top out at much the same as the Ultra II.

Has anybody else looked at this stuff? It appears in future I'll only be buying SanDisk Ultra cards.