time varying head condition

I have specified a cyclic time-varying head condition along a boundary to represent a tide (no density effect) with a time interval between the head values of 1 hour and a curve type "polylined".  I then output results to a dac file very 15 minutes.  In post processing I set an observation point on one node along the boundary.  If i compare the heads from the observation point with the one i specified they are different even at the specified points.  eg. say at t=0 h=0 and t=1hr h=0.232, observation point says t=0 h=0.0167 and t=1hr h=0.246. 

Thanks