I think this is natural. Whether his dick or the way he makes love to her is better in some way, who knows but even if it was the same or even a little worse, I expect she would still be excited by the novelty and taboo of it. Unfortunately, nothing kills sexual excitement like over-familiarity. I think this is especially true for women. Men tend to have a certain base sex drive and will just periodically want to fuck every so often, no matter what. I think women's sex drive is more circumstantial and requires more external stimulation. You are safe and secure as her life-partner, it is easy to take your attention for granted and assume it does not require as much effort, whether that's true or not. Thus she is likely to feel more drive to show off for the new man, to perform more for him to keep his interest. At the same time, being the center of attention for two men is no doubt a huge turn on. Even if she seems to be energetic in her sex with the other guy, part of it may also be a performance for you and I am sure she gets off to some extent knowing that her husband is watching her be a slut and getting off on it.
When its over, inhibitions and caution return - she underplays her interest because she doesn't want you to become jealous and upset. I think also it is just that the male "sex brain" is kind of always running low-key in the background but the female "sex-brain" is off a lot of the time and takes a while to warm up, especially once they have kids and their day to day thoughts are dominated by other concerns.