^^^Because a "perfect" reading shop-wise is a reading within spec.
As you can plainly see, that spec is allowed to go a long ways towards the negative side.
And, -2.3° is a boatload of camber out back, yet is "perfect" because it's within spec.
Did they give you a print out after the alignment so you can tell where you're at?
Really, unless you're racing, -1° in the back is as much as you'll ever need, and will certainly help with the inner tire wear problems.