Not sure if this is mentioned anywhere, but when you create a SMS Protected Distribution Point (PDP) with no boundaries, it seems to default itself as a Distribution Point (DP) for the subnet that it is located on.
For example, if the server has an IP Address of 192.168.1.25 /24, the server becomes a DP for 192.168.1.0 even w/ no boundaries defined. Until a boundary is defined, the server is just considered a DP for 192.168.1.0. When a boundary is defined, then the server is marked as a PDP. Interestingly enough, even if the defined boundary is 192.168.2.0, the server then becomes a PDP for both 192.168.2.0 and 192.168.1.0.
This is at least what I have experienced/observed from SMS 2003 SP3, the DPInfo table, and Swimage in SQL.
[Update]: This may be a Swimage related issue only. Swimage may only be looking at the IPSubnets column of the DPInfo table to determine which DP it should pull it's packages from.