Temperature, humidity, altitude, fuel load, mixture, ethanol concentration and wind and any other element I maybe forget have an effect, especially on a motorcycle with a narrow powerband like the Honda NSR150 SP. Up a steep hill the extra weight of a full tank of petrol can alter a motorcycle's power-to-weight ratio enough to be noticed. Wind is a factor at higher speeds and can easily add or knock off 30 km/h from a motorcycle's top speed.
Temperature, humidity and other elements affect a motorcycle's carburetor fueling. This is particularly significant on highly-tuned two-strokes. Get out on a cool, dewy morning and it should fly compared to a hot and more dry afternoon...
Some days my Honda NSR150 SP seems to run better than others but there's nothing wrong with it. So why does the performance vary? There's one particular hill that really shows up the difference.