Op Amp IC slew rate discrepancy
The slew rate is given as 'typical' 0.5V/us under the following conditions: 


Ta = 25 degrees C
Power supplies +/-15VDC
Gain = 1


Minimum is not specified except for the 741A, and it is 0.3V/us. 

If any of your conditions are other than as above you can expect some variation, and in any case the minimum is not guaranteed for the general purpose version (though I would expect it to be within 20-30% of that in most cases under the specified test conditions).