Why is the USD spread of the order book on GDAX kept very low most of the time? I noticed that there is a difference in the appearance of the order books among different exchanges. 

On GDAX, the USD spread is kept very low most of the time ($0.01). At these times, the buy and sell orders closest to the mid market price amount to a higher BTC value than what is actually traded.

This inherently leads to a trade history that is much more stable on a short time scale, which makes me think that it has something to with "manipulation" of the current BTC price, for example by use of bots.

Other exchanges usually have a much higher USD spread (in the order of dollars), and thus a trade history which is less stable over short periods of time. 

GDAX screenshot

(USD spread $0.01)



Bitstamp screenshot

(USD spread $12.11)



What is the reason for this discrepancy? What is the gain of forcing a more stable short-term BTC price?

Edit: Another screen shot from a moment ago. This is kind of ridiculous. Bots gone awry?
GDAX offers 0% fee to maker (traders who's trades are being matched by a seller or buyer). That incentives traders to try to match bid and ask as close as possible thus reducing spreads.