Many fixed/SS bikes use a 1/8" drivetrain vs. a 3/32" drivetrain (which is standard for road bikes). We use 3/32" for a few reasons:
Weight: 3/32" chains are lighter weight. Yes, it is a small amount, but every gram counts. :-)
Noise: 3/32" chains tend to be quieter - especially if the chainline is off - given they are more flexible than 1/8" chains.
Compatibility: There are a lot more options with 3/32" chains given that tis the same size as many road chains so you can use road chains as a substitute for single speed chains if you are looking for a particular look or spec. Also, there is backwards compatibility as you can an 1/8" chain on a 3/32" drivetrain (but can't use a 3/32" chain on an 1/8" drivetrain).
It works: 3/32" chains works fine for most riders and most don't need a 1/8" chain. Of course, we understand if riders want an 1/8" chain and we can work with them to spec a drivetrain that works for that chain. Just let us know.