Carthage doesn't appear to "see" latest commit of local repository specified with a relative path

This is kind of an obscure corner case. I have a Cartfile that specifies a dependency on a local repo like this:

git "../ISUtils" 

If I run carthage update I get

$ carthage update
*** Fetching ISUtils
*** Checking out ISUtils at "1.0.0"

I can change the Cartfile to specify the same repo using an absolute path

git "file:///Users/bryan/Documents/ISUtils"

Now if I run carthage update I get

$ carthage update
*** Fetching ISUtils
*** Checking out ISUtils at "1.0.1"

The second checkout is correct, the latest commit is tagged "1.0.1". The commit before that ("HEAD~1") is tagged as "1.0.0".