Why is there a difference in the voltage drop across the same resistor when using an oscilloscope and a multimeter?
There is no way to know which of these readings is more accurate unless you can tell us the uncertainty specifications for the two instruments. There are lousy oscilloscopes and great multimeters, great oscilloscopes and lousy multimeters.

If you want to determine the accuracy of the measurements you must consult the error/uncertainty specifications for the two instruments. Make sure that they have been recently calibrated using the manufacturer's recommended procedure. Make sure you are using the instruments under the conditions (temperature, humidity, warm-up time, etc.) specified by the manufacturer. Then, and only then, you can apply the uncertainty specifications from the manufacturer and determine the uncertainty (lack of accuracy) in the measurements.