Priced wrong.
Both of the big sites give a card a quality rating and never let price touch it, so you are told a card is good and left to work out whether it is worth the money. This is the other half: what the market pays for a card with this rating, these stats, these playstyles and roles, this rarity and these squad links, set against what it is actually listed at.
A gap is a disagreement, not a verdict. The model prices a card by its profile and knows nothing about who the player is, so a base gold trading far above its profile usually means the market is paying for the name rather than the card, and that premium is real while it lasts. The model lands within about 17% on a typical card, so anything in the hundreds is telling you about the model's blind spot as much as the price.
Trading under
Listed below what the market pays for the profile.
Trading over
Listed above it. Either a name premium the profile cannot see, or a card to sell into.