Why do Quantum Efficiency and Spectral Sensitivity curves for the same image sensor look different?

Both describe how a CMOS image sensor converts light into an electrical signal. They are often used interchangeably, but they are not the same. 
 
𝗤𝘂𝗮𝗻𝘁𝘂𝗺 𝗘𝗳𝗳𝗶𝗰𝗶𝗲𝗻𝗰𝘆 (QE) shows how efficiently incoming photons are converted into photoelectrons. It reflects the intrinsic photon-to-electron conversion efficiency of the sensor. 
 
𝗦𝗽𝗲𝗰𝘁𝗿𝗮𝗹 𝗦𝗲𝗻𝘀𝗶𝘁𝗶𝘃𝗶𝘁𝘆 describes the electrical response (typically in A/W) for a given amount of incoming optical power. 
 
The difference comes from 𝗽𝗵𝗼𝘁𝗼𝗻 𝗲𝗻𝗲𝗿𝗴𝘆. Photons at longer wavelengths carry less energy, meaning the same optical power contains more photons. Therefore, Spectral Sensitivity also reflects this wavelength-dependent photon-energy effect, while QE only describes the conversion efficiency per photon. QE is often considered the more fundamental metric for understanding sensor performance, while Spectral Sensitivity is useful for predicting system response under real illumination conditions. 
 
Understanding the distinction helps you interpret sensor datasheets correctly and make better decisions when designing an imaging system.