Arnold Zwicky over at Language Log has been looking into the phenomenon of “under” versus “in” occurring before the phrase “[modifier] circumstances”. I, personally, couldn’t recall ever hearing someone say “in the circumstances”, but Zwicky stated previously that not only is this how he says it, but it’s considered proper.
Of course, it’s Language Log’s purpose to debunk prescriptive language rules, so he did a little googling to see how people are actually using the phrase. Not content to simply check with “the circumstances”, he tried “these”, “all”, “no”, and several other modifiers.
In summary: the Google data suggest that “under” is preferred to “in”
with determiners “the” and “these”
with determiner “which”
with determiner “what”
with quantity determiner “no”
but that “in” is preferred to “under”
when “circumstances” means ‘personal situation’
with determiner “those” in general
with determiner “those” plus certain following relatives
with quantity determiners “all” and “some”
with quantity determiner “many”
with quantity determiner “a few”
This just scratches the surface of the phenomenon, but it’s enough to indicate that several effects are probably going on. As usual, the facts of usage are complex, subtle, sometimes surprising, and not easy to derive from first principles.