This is the picture I had open in photoshop at the time.
Here's the picture saved at 500 x 333 using very high quality (very little compression)...
Here the same picture, 500 x 333 using low quality (a lot of compression)...
As you can see, both images technically have the same resolution of 500 x 333, but there are huge differences in the two and it all comes down to compression.