Here's why:

* Caffeine's maximum absorbance is at 273 nm. This is where the molecule absorbs the most light, leading to the strongest signal and the most sensitive measurement.

* Using 275 nm is close to the maximum. While not the absolute peak, it's still a reasonable wavelength to use for measuring caffeine. It's often a practical choice, as instruments may not have precise settings for 273 nm.

* Factors affecting choice: The specific wavelength might also be chosen based on the specific instrument and its limitations, the presence of other substances in the sample, and the desired sensitivity of the measurement.

In summary: While 273 nm is the ideal wavelength for caffeine absorbance, using 275 nm is acceptable and often practical.