Mab keeps a reserve which was based at Arctis Minora, it’s this reserve which is balanced by Summer forces, otherwise Winter including its gate detachment vastly outnumber summer.
I suspect Mab deliberately moved her non-gate forces to flush out a suspected traitor. This appears to have been confirmed in the Novella The Law when Nameless Demi-god of Discord (not to be confused with Elon Demi-god of Twitter) and vassal of Mab since he claimed sanctuary from Kemmlers defeat (he was an associate, nit apprentice), was banished from Winter Lands following the attack by the Black Council to free nem-Lea which occurred just before Harry’s own assault on Arctis Tor to free Molly. Remember the winking ice statue? Mab never truly left.
This is why I suspect Nameless of being Cowl, Winter Law might prevent him from attacking Arctis Tor, but he is a lawyer, his vows likely were worded to allow him to pass on Winter secrets and maybe leave the back door open, but nothing more. No open evidence he colluded with the attackers which is why he still breathing, but enough circumstantial evidence for Mab to ban him. Very Cowl who is known to take major steps to hide his involvement and identity, and of course cowl has let slip to Harry he knew all about Bob containing the Word. Every other known associate or apprentice of Kemmler is now dead, the last being Corpsetaker in Ghost Story. Ergo Nameless is Cowl.