As Michael Karas notes, you are usin a page-mode chip. You can only perform an operation within the current page. And the pages have 256-byte align, so continuing to write past the end of a page will not result in a flush of the current page and a continuation from the start of the next page. Paged operation is common for EEPROM memories. In some part because an EEPROM is very similar to a flash memory, where you need an erase before you can perform a write. But also to retain compatibility with really old and small memories limited to just 256 bytes of total size. With flash memories, the pages (in that case normally named sectors) are even more visible.