There is another possibility, unsettling though it may be. The subframe, front or rear, might not be square to the body. The left and right rear toe needs to be the same number. The numbers you relate are incorrect. If your steering wheel is straight as you drive down uncambered road then your alignment has been done incorrectly unless the axles are not at 90 degrees to the centreline of the body. I generally test the quality of an alignment by driving down the middle of an untravelled road so that road camber steering effects are equal left to right. If the car tracks straight, steering wheel centred AND the rear toe is exactly the same left and right then the bodywork, subframes and suspension have to be correct. On the other hand if it tracks straight with different rear toe left and right then something is very wrong with the car.
Incidentally this is why the process us called tracking adjustment in the UK because that is the objective of the work. We call it an alignment because that is the process used to achieve correct tracking. The suspension is adjustable so as to allow the mechanic to compensate for minor misalignments inevitable in complex assemblies. If the adjustment range "bottoms out" then something is wrong with the complex assembly that is the suspension system holding up your car.
McPherson (Chapman if at the rear) struts have a number of advantages not least of which is very reliable alignment. Generally speaking no camber adjustment is required or provided for in a strut design because the tolerances can be quite large before any effect results at the tire contact with the road. Remarkably, caster angle is also fixed. This results from the very widely spread upper pivot point for the virtual upper control arm provided by the fixed point of the top of each strut. That point has to become way off before much change occurs at the tire. Frustrating for wannabe racers with strut suspension because providing for enough adjustment range to make any difference requires massive camber plates on top of the strut towers or resorting to highly questionable methods of cam bolts or even undersized bolts at the hub end of the strut where the adjustment has greatest effect.
My 164 with very worn suspension (high mileage) has not required an alignment for many years. My SAABs were all the same, hardly ever needing an alignment although the rear axle was non adjustable being a dead beam design, very rugged.
Unlike the front axle which always splits the toe equally because of the steering rack being free to move the rear toe has to be set individually for each axle. If toe is uneven across the front axle then the the steering wherl goes off centre. If the rear toe differs between wheels then a thrust angle results. The symptom of that is again the steering wheel will go off centre as the driver compensates for the thrust forces from incorrect alignment. It is possible for an incompetent shop to get the alignment wrong at both ends in an incorrect attempt to centre the steering wheel.
The explanation given by your shop just cannot be correct.