I'm not a native english speaker, but my impression is that whenever names are sorted in such a way that the definite article "the" in the beginning of a name is ignored, then the indefinite articles "a" and "an" in the beginning of a name are also ignored. This is how iTunes works for example.
However, they way this seems to work with movie names in Infuse 6.1.5 is that the sorting algorithm ignores the prefix definite article "the" but not the indefinite articles. I'm requesting you to change the algorithm to be in line with how article insensitive sorting typically works and ignore prefixes "a" and "an" also.
I did some further (minimal) research into how movies are typically sorted by title, and it seems it's not as clear cut as I had assumed:
Ignores "the" but doesn't ignore "a" or "an" (the same as Infuse currently)
Ignores none of the articles
All of these ignore all articles: "a", "an", "the". Additionally, in iTunes and Plex the sorting behaviour is configurable (but by default all articles are ignored).
Based on this small sample, I would still lean toward changing the sorting algorithm in Infuse to match Plex and iTunes both because it appears to be the most common way to sort and because to me it seems the most sensible thing to do.