Ambiguity in a contract is generally held against the party that wrote it. Do you have anything in writing where they agree to the 1 month penalty and retention of security deposit? I think at best a court would award you the 1 month breaklease penalty but doubtful they would let you keep the security. Every clause in your lease needs to be clearly defined for the tenant will always interpret clauses to their benefit when ambiguity exists
My guess is they're saying "Since I'm going to court anyways, might as well file a counter-suit to get the one month back for the inconvenience" or something along those lines. I don't think they feel like they can win the counter-suit, but might as well try since I'm taking them there anyways. Just my thoughts.
If i were you i would try to keep this out of court. They could win their countersuit depending on the Judge you get. He could try to "teach you a lesson" for handwriting in a poor lease clause. Judges also try not to award security deposits to be used as any kind of penalty fee. I would tell them I'll drop the suit if we go back to our agreement of 1 month and the security deposit. And remind them you have proof of their agreement to that settlement.