For targets smaller than resolution, range drops like (size/res)^2. Your scanner has 166MKm vs. size 100. You say the range is 77 MKm, which is ~1/2 166MKm. So anything smaller than sqrt(1/2)*100 or ~70 Hull spaces will be missed by your scanner at that range. (Assuming I've read your screen shots correctly.)
My guess is that they're simply small targets. For example, against a size-20 FAC, your res-100 scanner will only have 4% of the range, or roughly 6.6MKm.
John