Ah yes, the good old non-linear addressing architecture of the x86. Honestly, it's been too long since I dealt with this stuff for me to be sure I can give you a good answer, so I'll finish this with a link to the wikipedia article about it.
A couple of slightly modified quotes from that article:
"In Real mode, the 20-bit segment address is always added with a 16-bit offset to yield a linear address, which is the same as a physical address in this mode."
"For example, it can be used to advantage when addressing multiple nested data structures."
The key word here is 'overlap.'
en.wikipedia.org/wiki/X86_memory_segmentation
