Probly not a fluke, Cats don't just get better, the O2 sensor is probably compensating the fuel and air better since its not bad. I would still replace the Cats. I asked about the mileage b/c that would determine whether or not the O2's actually killed the cats or not. In your case I don't think so, b/c 100 miles really isn't enough to clog a cat, not to mention they are two seperate(sp?) cats with two different sensors. Like I said before OEm cats usually only last to about 120K miles anyway.
NacIK-- I think you might have a couple issues, sounds like cats plus a coolant leak somewhere, IF the leak is not on the outside its definiitely in the motor. Coolant just doesn't disappear it goes somewhere, the intake gaskets are known to leak around 100K and the head gaskets would be my next guess.
Driverside_ yours is probably the intake or the head gasket as well, like i said coolant just doesn't dissapear.