I have recently heard a lot of debate about how silver stain works. There
appears to be no consensus. However it is widely, if not totally, accepted
that silver works on the surface of the gel leaving the core of the spot
un-stained. Your experience would appear to be in line with this. Another
debate is whether, when you digest the spot with trypsin for mass spec
identification using mass mapping, you actually get effective digestion of
the stained surface or whether you only derive peptides from the unstained
middle. This has obvious implication for parameters such as thickness of
gel etc.
In short I think your observation is quite well known.
Paul Cutler
SmithKline Beecham