Severus had had a few years to adjust to being able to tell the truth on a regular basis. He didn't do it gratuitously, still - there were plenty of things he saw no reason to bring up even if they weren't strictly off-limits, and a few that he still kept as close as he ever had. But he had more or less grown accustomed to the fact that there was no longer any grand, over-arching reason to say as little as possible. What was new and unfamiliar was the fact that now there was actually someone to whom he might care to say anything. Dumbledore, Malfoy and the others he'd been (somewhat) open with during the war were a different phenomenon. He couldn't imagine wanting to chat with any of them. The urge to go beyond the barest bones of pleasantries with someone with whom he had no pressing business was peculiar enough that it couldn't help but be striking.
Which made him wonder, now, why it hadn't quite stricken him before, but. It was ridiculous to delve into all of that over this, surely. First of all, he wasn't going to tell her what he knew about Goyle, at least not the more sensitive parts; in that he considered himself to be doing her a bit of a favor. (Who would want to know? It would be better if he didn't spend too much time thinking about it himself, because the thought of it still threatened to make him livid.) And then, he did owe Goyle something, and he wasn't going to risk having all of those careful plans upset by something that could see him tossed out, however unlikely she was to spread rumors.
The uncomfortable truth was that whatever he had said or insinuated, he had done it almost entirely because he wanted to talk to her, and that really ought to have been unacceptable. But he found he didn't really care. He was almost certain that it was no great sin to want to talk to someone, not anymore. Especially when they were ... well. Pleasant. And so he waited in his office, as casual as he ever was, standing at one of the shelves and thumbing through one of his collections of treatises on hellebore. He could distract himself as he pleased.