Oscilloscope voltage readings factor of 10 too high. Is this due to the impedance?
Some of the probes used with oscilloscopes incorporate a very high impedance input circuit right at the probe tip. This circuit attenuates the signal by a factor of 10. Some oscilloscopes can automatically recognize whether such a probe, usually called a "ten-to-one" probe, is in use but other oscilloscopes require the user to manually change a setting.

My guess is that your oscilloscope assumed that such a probe was being used when in fact it was not, so the recorded voltages are too large by a factor of 10.