SD Card Speeds Up with More Data on it?

I recently bought an SD card to use to move data between my computers at home and on campus. I was a bit dissapointed at the speed of reading and writing to the memory. It took hours to upload all my data onto the card the first time around. But now I noticed that files seem to copy MUCH faster. I put this down to the data on the card now being organized and, therefore may be more of the faster "