Though they may be All Terrain Armored Combatants, ATAC are not perfect. Each ATAC type handles differently on different terrain, meaning certain ATAC require more AP to move across some spaces. To check the terrain of a certain space, move the cursor over it and read the data in the bottom left corner of the screen. Of course, if you have a Dormant Ability such as Snow Motion, Forest Motion, Desert Motion or Hoverjets, the AP costs for snow, forests, deserts, or all (respective to each ability) will always be 10 AP per space.
Below is a table with terrain types, as well as data on how each ATAC type fares against it. Please keep in mind that the first space that you move costs DOUBLE the AP cost, and any ally-occupied square costs 5 extra AP to move across. And for reference, here are the ATAC types and the ATAC that make up each type.
Infantry (Normal): Alba, Altagrave, Amphisia, Andoras, Barazaph, Barbatos, Crimson, Eigol, Einlager, Glaive, Haizuron, Haurol, Rahabor, Ratatosk, Serata, Solarus, TIC-TAC, Ultragunner, Yurangol
Light: Dantarius, Flaros, Sarbelas
Heavy: Bahamut, Roaring Lion, Sharking, Zulwarn
Ninja: No. 69, Vedocorban
Flight: Korbelan, Sylpheed, Waiban