I can only suggest that the previous owner didn't want to splash out a new seat post of the correct size and used a shim to reuse a post they already had.
@yoav started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
I can only suggest that the previous owner didn't want to splash out a new seat post of the correct size and used a shim to reuse a post they already had.