The two main reasons why the lower corner of the biopotential frequency response is different between the ECG and EEG signals (as called out in DSI Guidelines for Biopotential Applications) are:

1. Differences in tissue impedance contacting the leads.
2. Surface area of the lead material exposed to the signal.
   
   This is an assumption based on the surgical recommendations of tacking down the lead to the muscle for the modified lead II position recommended for rodent ECG where more of the lead wire is exposed vs the smaller contact area the leads make with the brain through the hole drilled into the skull.

Of the two reasons, DSI believes #2 to be the more significant contributing factor.