There are various options for the wheel sizes including both diameters and widths, the rear driven wheels tend to be wider than the fronts and therefore the tyre tends to be wider. To ensure the rolling radius is the same with different tyres on the front and rear the aspect ratio (35/40) is different.
This is known as a “Staggered Wheel” set up and is common amongst more “ sporty” cars.
Looking at the list below 19x8 is a 19” diameter rim 8” wide, the next option is 19 x 9, same diameter but 9” wide.
So the reason why you have different tyres is probably because you have wider wheels on the back, what are the other figures off the tyres, the first 3 give the tyre width in mm. For example 245/40/18.