32-bit processes on 64-bit Windows run in a layer called WoW64 (Windows-on-Windows64). If you capture a mini dump of such a process from a 32-bit debugger, you end up with a register context for the 64-bit WoW64 process rather than the 32-bit one you probably care about.
This detects WoW64 by looking to see if there's a module named wow64.dll loaded. For such processes, it then looks in the 64-bit Thread Environment Block (TEB) to locate a copy of the 32-bit CONTEXT record that the plugin needs for the register context.
I'll add a test before submitting this, but it seems to work in my manual experiments.