Imported iPhone Photos have different pixel dimensions than the original photo

Adding to Matti Haveri's analysis: Photos always keeps the original, the 4:3 sensor data. An important point is that you say that you "shot all my photos 16:9," but you actually shot them at 4:3 and you told the phone to cut off parts to change the aspect ratio to 16:9. But Photos always keeps the original in case you want to change something later.

When you transfer pictures from the phone, you have to decide if you want to transfer originals or edited versions. (Matti points out that this decision may be made for you in your choice of transfer method.) Transferring both versions will take twice the storage. On the other hand, if you use iCloud to synchronize your Photos Library, then all the data, original and edited, will be transferred taking little more storage than the original, and you'll see the 16:9 version in the Photos display.

Obviously 8064 × 6048 is the original 4:3 version and 8064 x 4536 is the cropped 16:9 version that iPhone creates automatically if you shoot 16:9 (the sensor is 4:3), and your workflow imported only the original (importing via USB does that as well as an extra AirDrop option, for example).