Suggest you check your power curve. How long a time did you really average 5.5 for? It’s easy to fool yourself by taking quick glances at just the right time. I suspect at 5.5 you were accelerating relative to the others, but maybe not for long enough to notice gaps changing.
Even on slopes lightweights need a bit more W/kg (your bike is a larger proportion of your weight) but it should only be about 0.2