Negative Decoupling at 175 Watts

My 225-watt test showed 8% decoupling in a 30-min test. (later determined that it wasn't really 8%, more like 3-5% if I used the correct calculation method). I wanted to see the results of a lower wattage test, so I did an 80-min decoupling test at 175 watts.

Not only did I not find any evidence at all of decoupling, the result was NEGATIVE 2%. First 37 min: 176w at 123 bpm; Second 37 min: 177w at 122 bpm. Go figure.

I'll do the 225w test again to see if I can repeat the 8% decoupling result. If I can, then the next step will be to see at what wattage the decoupling effect starts to occur. Will it be at an abrupt wattage or will it gradually decouple?

If I find that it's an abrupt point, will additional aerobic system training raise the 'decoupling point?' Will additional aerobic training lower the 8% decoupling I see at 225 watts? Lots to investigate; but that's ok - I have until Spring.