Web ads with drastically different clarity, apparently same specs

I just posted an ad with photo and text to an online newspaper, and the specs required were really low. They wanted a jpeg, 300x600pixels, no larger than 75 KB. The text looked predictably awful, so they said their art department would reduce it for me, but it looked just as bad. There is small text, often in a color, on another solid color, with some white text on a solid color. In rotation with my ad is another, that has 6pt type that looks immaculate. Some is dark on white background, but some has white text on a solid color, much smaller than mine. Their art department swears the file specs are the same, but I have never seen a jpeg that crisp. Any ideas what the secret might be? Thanks!