Wheatstone bridge comparator?

ok, so a wheatstone bridge is set up as two voltage dividers consisting of two resistors each, with each divider parallel to the other. when the bridge is balanced the voltage between the two center points of the dividers should be zero. so when i connect a 741 op amps input terminals, one in the center of each divider network, the difference between the two input terminals should be 0V. now, if an op amp is supposed to amplify the difference between its inputs then why does a balanced bridge drive it into high saturation? and other bridge conditions (or so multisim seems to tell me, along with me schoolbooks) drive it into low saturation?? it seems to be doing the opposite of what i understand an op amp to do. what am i missing??