From pressure to SPL : microphone calibration using sound level meter calibrator and sound level meter
First, I would check the positioning of sound level meter and microphone. If you are putting them close to the oscilloscope a few centimeters of displacement may change the measured value dramatically. (6dB/doubling of distance)

Second, which microphone do you use? The link you provided leads to a preamplifier. The actual capsule can either be a free field or a pressure field one. A free field capsule should be facing the sound source directly, while a pressure field capsule should face it sideways.