This seems to be a card-specific issue (not set), where the Steam browser was 6X the regular amount for that set, and the third-party was only 1/2 the regular amount.
Steam has an error/bug where they can offer different amounts of gems based on your browser, with the Steam browser getting more than others. It's far from a complete list and may not be accurate because Steam lowers values without notice (most likely as a function of base price). This page is an attempt to document the value in Gems of cards, emoticons, backgrounds, and booster packs of various games.
