Mine is -3 to -4 on level surface.
I went to the Apple store try to get an exchange, but then confirmed that they will not be replacing it with a brand new retail phone, but their typical "new" phone in a plastic non-marked box.
For those of you who may care about unlocking your phone and use it abroad(I have AT&T), I had my iPhone 4 replaced under warranty with those new/refurbished phone in black plastic box, and when my 2 year was up, AT&T said they have no record of the phone and could not help me unlock it, even after providing the IMEI and ESN#. When I contacted Apple, they told me to call AT&T, so I ended up with an phone that's locked even after contract expiration.
So I decided to wait for Best Buy (where I bought the iPhone 5s) to get a replacement and get it replaced with a new phone.
I wonder if it is specific to a particular batch of the iPhone 5s, whether it's 16/32/64gb or a particular color model or a particular series of serial number batches.
For what it's worth, my phone is space gray 64GB AT&T iPhone 5s. And when I entered my serial # into the link
http://www.chipmunk.nl/klantenservice/applemodel.html , I get the following result:
Nice Name: iPhone 5s
Machine Model: iPhone6,1
Family name: A1533
Model Number: ME311
Group1: iPhone
Group2:
Generation:
CPU speed: 1.3GHz
Screen size: 4 inch
Screen resolution: 1136x640 pixels
Colour: Space Gray
Production year: 2013
Production week: 34 (August)
Model introduced: 2013
Capacity: 64GB
Memory - flavour: xx
Factory: DN (China, Chengdu - Foxconn)