I created a new file with 300 dpi and 10,7x15 zentimeter, which calculates to 1264x1772px. When i export it as .jpg it saves with 1264x1772px but has 96 dpi. When i export it as .tif however it has the correct dpi, so there doesn't seem to be something wrong with the .psd file.
Well thats the point. I designed a flyer which has to have a certain dpi and zentimeter size and they wouldn't accept my files because of wrong sizes and i couldn't figure out why until i realized that the exported jpg only has 96 dpi.
I used alt+ctrl+shift+w (export as, don't know exactly how it is called in english) and chose jpg (without recalculation?).
Try just using File > Save As and in the pulldown menu where it says Format > JPEG. It'll default to Photoshop most of the time so you'll need to choose JPEG. It will let the source dimensions stay as they are.