There's basically a percentage adjustment that the ECU makes to the fuel curve as its learned value. When you disconnect the battery, it "forgets." This is what resetting the ECU does is puts it back to forget mode. Until you get it warmed up and driving in closed loop for a while, your engine might run like this. My '90 does something like this, and driving it around nicely in closed loop mode for a half hour or so gets it straightened out.
This does, however mean that something is a fair amount different from that the TCCS is expecting, which may indicate a problem. I'm pulling the motor in my '90 really soon, and at present, things seem to be fine, so I'm ignoring it for the time being. If it continues with the new motor, I'll start a fairly rigorous fuel system diagnostic.