Serialization issue. From the Introduction to Cap’n Proto:

"Cap’n Proto is INFINITY TIMES faster than Protocol Buffers. (...) there is no encoding/decoding step. The Cap’n Proto encoding is appropriate both as a data interchange format and an in-memory representation, so once your structure is built, you can simply write the bytes straight out".

I take it as a rationalization of what OLE Compound File Binary - internal Microsoft Office memory structures serialized "raw" as file format - would look like if they paid more attention to being backward and forward compatible and extensible.