the first version of the 1D has a sample rate of 50hz. for all subsequent versions and other Shake models, data is delivered at 100hz. due to this change in sampling rate, and because of how ADCs work, this offset is, in fact, necessary.
normally, when processing raw seismic data, software will demean the data to remove any offset which may or may not exist since there is no guarantee that incoming data will be centered around zero.
if you want to demean this data yourself, you can take the average of the data points over some period of time and remove this value from all the data points in the time window of data being processed.