Why is this exit indicator placed here?
You dont have this tagged with a jurisdiction and the answer may vary a bit by country regulations but here in the US under the FAA the requirements do not explicitly state the sign must be in the middle of the aircraft: 


  § 25.812 Emergency lighting.
  
  (b)Emergency exit signs -
  
  (1) For airplanes that have a passenger seating configuration, excluding pilot seats, of >10 seats or more must meet the following requirements:
  
  (i) Each passenger emergency exit locator sign required by § 25.811(d)(1) and each 
  passenger emergency exit marking sign required by § 25.811(d)(2) must
  have red letters at least 1 1/2 inches high on an illuminated white
  background, and must have an area of at least 21 square inches
  excluding the letters. The lighted background-to-letter contrast must
  be at least 10:1. The letter height to stroke-width ratio may not be
  more than 7:1 nor less than 6:1. These signs must be internally
  electrically illuminated with a background brightness of at least 25
  foot-lamberts and a high-to-low background contrast no greater than
  3:1.
  
  (ii) Each passenger emergency exit sign required by § 25.811(d)(3)
  must have red letters at least 1 1/2 inches high on a white background
  having an area of at least 21 square inches excluding the letters.
  These signs must be internally electrically illuminated or
  self-illuminated by other than electrical means and must have an
  initial brightness of at least 400 microlamberts. The colors may be
  reversed in the case of a sign that is self-illuminated by other than
  electrical means.


So if its easier due to electronic cabling, head room, brackets, or any other factor manufacturers are free to mount the sign off center.