When /entitydata is used with an entity selector that contains dx, dy and/or dz, it seems to only change the data of one random entity in the specified range.

Example: Running

/entitydata @e[0,11,0,dx=49,dy=0,dz=0] {Item:{id:"minecraft:stone"}}

on a row of 50 wood items results in this:

(This is the only item that was changed to stone). For the items that weren't changed, it outputs "The data tag did not change: [...] id:"minecraft:stone" [...]", even though they're still displayed and handled as wood items.