Need some explanation for the filter parameters for filtering a signal

Hello, I am using the filter as shown below

d = fdesign.lowpass('Fp,Fst,Ap,Ast',3,8,1,20,500);
       Hd1 = design(d);
  length=length(Hd1.Numerator);
  data1 = filtfilt(Hd1.Numerator,1,data1);

And it is filtering the signal as shown here

But I got some doubts regarding its working

1)if Fs=500Hz and length(filter length)=104, then the FIR filter can only see 104/500Hz = 208msec worth of samples. It cannot see one full cycle of any frequency below 1/208msec = 4.8Hz. But I don't understand why it is working fine for the freq below 4.8Hz allowed by the lowpass filter.

2)when the all the filter coefficients are added like this

s2=sum(Hd1.Numerator); it is giving 1.0559

It means that the output of filter is almost 105% of the input when the frequency is less than 4.8Hz.

Can someone explain why the filter is working fine for the freq below 4.8Hz allowed by the lowpass filter when it is not seeing any full cycle of freq<4.8Hz.And also any other information regarding the working of filter.