There are all kinds of different rear spring combinations, I wouldn't be surprised at this amount of variation. The 27" is just an approximate dimension. The 165 tire will make the car sit about .3" lower than the original tires.
The lower ride height probably contributes to a better ride than the 27" measurment by giving a little more rebound travel which these cars really need.
Of course the down side is that there is less ground clearance & if the rear is too much lower than the front it may give a tail dragging appearance. Probably better to be a bit low than too high, which definitely makes the ride worse.
If the tail down appearance is a problem, you could lower the front to match by putting 1/2" thick tubular spacers between the front spring pans & the lower A arms. This would lower the front by about 3/4 to 7/8".
I'm sure that there are a lot of them out there that have less than the "spec" ride height. If it isn't bothering you, don't worry about it.
D