Naira - our identity

I was in bank of late, had to catch some grand to make up for my accommodation fees in school. After spending endless minutes waiting for the person to check my account balance to come on seat from wherever he left his responsibility for, and finally getting through, I joined the long queue towards the counter.
From where I was on the line I could see people been paid with some very dirty notes of 100 naira. Some were like complaining bitterly, others grumbling, while some others took it complacently and humbly walk away. I didn't really see how dirty it was from that far but when I got to the counter I had to cover my nose for some seconds. I can't help but wonder where on earth that kind of dirty, stinking notes could come from. The woman on the counter seems to get me.
"Hey, this is the only note I've got to pay you with", she said in a rather gruff voice.
For some fraction of second, I couldn't shift my gaze from the notes, wondering if I should simply walk out the bank or collect it. But I had no option. The payment I had to make was urgent and I have no other choice. A glimpse at the other counters shows them all paying out the same dirty notes.
The funny thing is that there on the counter is this CBN bill that goes "Don't stain, squeeze, tear, spray or write on the naira. It is our identity".
Despite the ads, bills, jingles flowing around about protecting the image of the naira, I wonder why a bank - a giant one for that matter (save me the name) - can pay out in such stinking notes. But come to think of it, where do they get the notes? Hey, I didn't say CBN! I only asked a question!