Fluctuating? No, it's very clearly defined and it stays the same instead of moving back and forth. It does move north about 15 meters a year (says wikipedia), but that's a steady movement, not a fluctuation.
If it moves, that means it fluctuates, doesn't it? Fluctuation doesn't have to be random. The reason that that caveat is included is because the nation of Chad sometimes has the tropic of Cancer passing through the northern portion of it, sometimes it doesn't.