Every dragon in Bestiary 1 that has a "Space" entry in its stat block shows reach for the bite (except for the Young Black, Ancient Brass, and Young Bronze, which don't have a space entry at all). All of the Medium sized dragons in the Bestiary 1 have reach (except for the mentioned Young Black that doesn't have any Space/Reach entry).
Most of the Large dragons in the Bestiary 1 (other than the Adult Brass and Young Bronze) have a listed reach of "10ft with bite." If, as the general rules for dragons states, that is the reach of a creature one size category larger than the dragon (i.e. Huge), then a Medium dragon would have a reduced reach (5ft) unless specified otherwise.
You should view the rule in the dragon section as applying to all dragons and stat them accordingly, not just blindly follow what is on a third party site (no matter how awesome and useful that site is).
If you're referring to the information I posted from the Draconimicon, then I agree, you shouldn't blindly follow it. But the table Matrix Dragon posted is actually in the Pathfinder Core Rulebook, page 195 of the 2009 hardcover edition, as well as the official Paizo PRD (Combat section), along with the note:
*These values are typical for creatures of the indicated size. Some exceptions exist.
Bold text is my emphasis. According to that table, medium dragons should have a reach of 5ft with their bite, unless specified otherwise.