Unwanted offset attenuation in FIR filters

I see, that the original (unsigned) filter design implements a DC gain of 0.57 (sum of integer coefficients = 1167, scaling factor = 2048). This is, what you see as an offset in the unsigned SignalTap. The input data is centered to midscale (8192), the output data to about 4650.

If you use signed data representation throughout the design, the data should stay zero biased when passing the filter, but still attenuate the signal level. The arbitrary gain is apparently a result of the FIR compiler's scaling method, depending on the number of coefficients.