Temperature, Humidity, altitude, fuel load and wind all have effect, especially on a motorcycle with a narrow powerband like an Suzuki RGV250. Up a steepish 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 15km/h from a motorcycle's top speed.
Temperature and humidity affect a motorcycle's carburetor. This is particularly significant on highly tuned two-strokes. Get out on a cool, dewy morning and it should fly compared to a hot, afternoon.
The difference in peak horsepower on the average stock 250cc engine is about 5hp. When a motorcycle manufacturer builds a bike, they usually install jets in the carburetor that are too rich. The manufacturers sell the same model worldwide. They couldn't afford to install different jets in the carburetor to suit all the different climates and types of fuel. In addition to the climate and fuel you must consider things like the terrain and type of riding. And then there is the most important jetting consideration, the rider.
For more about fine-tuning your jets read some story we posted some time ago..
article.php/Carburetor-Tuning-to-Rich-or-to-Lean
By the way, the Suzuki RGV250 is a great bike, with its unmatched 60 horsepower, from what year is your motorcycle?
- 
Suzuki RGV250 performance
