Besides that reason, I always saw it as mutual respect from two ex-office workers who worked under terrible conditions and survived. Happy they're doing better now.
Always love these small tidbits like these. Like how Miko and Fubuki calls each other as "Fubu-san and Miko-san" how Luna calls Kanata "Amane-san/chan" and how even though Lui is the most junior in terms of debut date she is referred by others as "Lui-nee".