It is not a matter of which to trust. This is the expected behavior. No image format saves the RGB intensity of every individual pixel. They use different methods to efficiently compress the image data so that the original image can be constructed from the compressed information. The purpose of image compression formats is not to save pixel-level information; rather, they are designed so that the overall appearance of the image remains the same.
JPEG format uses lossy compression: https://en.wikipedia.org/wiki/JPEG, which implies that if you compress an image using JPEG, some of the information in the original image is permanently lost. On the other hand, PNG is a lossless compression format: https://en.wikipedia.org/wiki/Portable_Network_Graphics, it retains all the information in the original image. Since PNG is a lossless format, therefore if the same image saved in JPEG and PNG, then the PNG file will have a larger size.
As you can see, the JPEG was never intended to save pixel-level information, therefore, If you compare both image pixel by pixel, you can see large variations at some points, but, overall, the appearance of the image is same. It is a tradeoff between retaining the image quality and making a file size smaller.
