. Filament material. Some materials glow brighter than others. Frequency can be important as our eyes are more sensitive at certain frequencies. . Filament shape, length, and construction can make a difference. . The biggest difference is probably just normal variation inherent in manufacturing a cheap product.
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The wattage is how much power they consume. If they are designed very well they will have the right material, the right size and shape filament to produce the most light. If they are not well designed they (the manufacturer) just stuffs in 7 watts of the cheapest wire that will provide some useful light. Also, If the bulb is coated on the inside different materials and thicknesses will tend to block different amounts of light.