Not all pieces that moved out of back rank are developed - for example in IQP positions a common mistake by begineers is playing Bd2, thinking that they developed their bishop (they didn't, bishop was working just as good on c1, so they wasted a tempo).
Alternatively something in position changed, so now one side's piece layout makes much less sense - for example in Sicilian Scheveningen common idea is to play Bd7-Nxd4-Bc6. If white moves his knight away from d4, thus preventing Nxd4, bishop on d7 isn't doing anything - it is practically undeveloped, and needs to be redeployed soon. Or maybe center got closed and suddenly one side's pieces are on "wrong" side of the board, while the advantaged side is ready to attack.
Or maybe the pieces are just too exposed. For example in Classical sicilian line 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 Nc6 6. Be3? bishop on e3 is too exposed, black can take advantage of this with Ng4. At first glance white has a small advantage in development (3 pieces out vs 2), however after next few moves he will either have to sacrifice a pawn for dubious compensation or waste few tempi with the bishop, giving black lead in development.
Development is not just a question of how the squares on which pieces are look - you also have to consider, if they are doing anything and if they are established securely. This of course strongly depends on particular position, so please add some examples to your question if you have them.