So different circumstances can correspond to different motives for acting, which is to say different objects of acting, and objects specify human acts, so different circumstances can correspond to different species of human acts.
If my motive is "to consume food," then my human act is "eating." If my motive is "to consume food" and the food I consume is costly, then my human act is "eating" and the circumstances include "sumptuousness." If my motive is "to consume costly food," then my human act is "eating sumptuously."
As Zippy is forever pointing out, there are no human acts apart from human willing. "Eating" and "eating sumptuously" are different human acts.
And it's not an abstract, grad seminar in philosophy difference; still less is it a rhetorical fiction that must be maintained so we don't have to scrap our whole moral taxonomy. It's a difference as practical and fundamental as the difference between your ox and thy neighbor's ox.
The trick is to make sure you're distinguishing acts according to motive rather than intention. This shouldn't be too hard: it's the difference between "what" and "why," although the fact that in common usage "motive" is used to mean "intention" doesn't help. (For what it's worth, Merriam-Webster Online defines "motive as "something (as a need or desire) that causes a person to act," which is a lot closer to how St. Thomas used "motivus" than to how the word is always used on TV cop shows.)
The practical difficulty in figuring out whether you're specifying an act by motive or by intention doesn't lie in the conceptual differences between motive and intention, but in fallen human nature. There are times when the only means to a good end that I really want are evil, but I really want that good end, so my intellect obliges by convincing itself that the good end I want is the real object of the means, and if the object of the means is good then the means are good (since, what do you know, the intention of the means is good).