Probably because when it is creating the new JPEG file it is using a lower level of compression. Personally I would try to avoid exporting, importing, and exporting again using JPEG files. Although even when working at maximum quality levels repeated saving of JPEG files introduces progressively more JPEG artifacts. This will even happen at maximum quality, where there is essentially no file compression applied. This is because you start with an image containing RGB triplets, which are converted into a different data format for storage in the JPEG file, one that instead records brightness, and chrominance, just like the old analogue TV signal did. When you then open the JPEG file, the data is converted back to RGB. Each time you repeat the change of data format you add some conversion errors, even if you don't actually reduce the amount of data by using the addition of compression.
If you have the RAW files I would be moving those over, and if not then I would be importing the original JPEG files to the new program, not exporting and importing. I thought that Apple provided a seamless method of importing your old Aperture library to the new Photos program, so that you did not need to worry about this issue. It would take all your old original files, along with the edit instructions, and add them to Photos. This would have been my prefered method of moving the data over. I know that they did the same for moving to Lightroom, so cannot beleive that you cannot do the same with their own Photos program.