Usage of も in そんなこともないけど

Question

Context: a guy has invited his girlfriend over to study, and the girlfriend finds a dirty magazine. The following exchange takes place:

男 : 香奈さん? 今日はテスト勉強しにいらしたのでしょう？
女 : 何が勉強だし! どうせ千秋はこういうことばかり考えてたんでしょ！
男 : そんなこともないけど…

Why did he say も here instead of saying そんなことないけど or something similar? I can't quite figure out in what sense the particle is being used. It doesn't seem to me a case of it being used in the sense of "even," or trying to show how big of a thing something is. It also doesn't immediately make sense to me for it to be in the sense of "also"; in that case it would mean "that's not the case" (in addition to other things not being the case), but the only way I can make that make sense is if I assume what he means is "I didn't invite you over for that, but I may not have invited you over to study either."

Answer 1

The も is used in the sense of...

も 🈩〘副助〙*　
⓬ さりげなく取り立てて、文意をやわらげる。...
㋐《多く、文末表現に使って》一歩引く気持ちで、その判断をやわらげる。...
「その件なら知らなくもない」「そんな感じがしないでもない」
(from 明鏡国語辞典)

The も softens the statement. 「そんなこともない」 sounds softer and less decisive/assertive than 「そんなことはない」.

_{* 明鏡国語辞典 categorizes 係助詞(binding particles) as 副助詞(adverbial particles).}

Answer 2

I will logically strenghen ǝʇɐןoɔoɥƆ's answer.

• Ａだ。 It is 100% A.
• Ａもだ。Ｂもだ。 It is 50% A.
• Ａもだ。Ｂもだ。Ｃもだ。 It is 33% A.
• Ａもだ。 It is less than 100% A.

So, the も softens the statement. "Ａもだ" sounds softer and less decisive/assertive than "Ａだ".