I am using 2.8.4 Pro. A user of 2.7.3 started downloading a file from me. The monitor window says:

Size 2,623K
Status complete
Progress 50%

How does that work ?

the person might not have wanted the whole file. they progressed 50% into the file and then said, "ok, that's enough."

that's basically it.

I had assumed that the status "Transfer Interrupted" was an indication of a transfer being interrupted by the recipient before completion.

that could mean the same thing, but it means the person didn't tell you they were finished. interupted could also mean the person just decided they didn't want the file anymore and stopped short. the difference is very small, but there is one.