The stratum protocol existed for some time earlier than BIP numbers had been ultimately assigned. Presumably there was some dialogue that they need to have BIPs and so BIP numbers had been assigned. Stratum began as Electrum’s protocol, however Slush realized that it might be used for mining, so the unique protocol was prolonged. Thus two numbers had been assigned, one for the unique protocol, and one other for the mining extensions.
Sadly, whereas the numbers had been assigned, the paperwork themselves weren’t really written. This was because of the authors not having sufficient spare time to take action, and presumably writing such paperwork was not a excessive precedence. The BIPs stay unwritten to at the present time.