I didn't investigate how does the actual monitor work, just looked at the listings.

But as long as there's no memory qualifier, SDCC puts declared data into memory space based on the memory model set in command line, which is by default the direct-addressable DATA space. And that's much less than 256 bytes - it's the lower 128 bytes less registers bit data, overlayed data (which is just another form of local/automatic data) and maybe also something else I forgot.

JW