It's nice to see Miho in middle school, a period that isn't so much as hinted at in canon, so we have no real idea what happened between Little Army and when Miho arrived at Kuromorimine.
Of course, Miho and Erika hardly seemed like how I imagined them.Miho never struck me as the tomboy type, nor the kind who'd be so familiar with Erika. As for Erika, my personal headcanon was that she might have been willing to accept that Miho was a better choice for Maho's second in command, at least before Miho saved the tank and cost them the match.
Then again, part of the appeal for Erika/Miho seems to be the "cracky" nature of the ship. The first Erika/Miho fic I saw was "Girls und Panzer: Hope Dies," in which their being in a relationship is a major surprise. In that fic, Erika says she initially hated Miho for costing Kuromorimine the victory, but eventually forgave her, and they eventually started a secret romantic relationship. Of course, that's not nearly as unbelievable as Yukari murdering Miho for rejecting her love confession in that same fic.