It is 2016, my team is using Crucible for 2 months now and i have just realized this problem. I am sad that i found this issue open and another (CRUC-6312) already closed unresolved!
So to catch up on this problem, i try to explain it from my point of view:

When i modify a file which exists in the file system for ages, then i have a start point. Then i do some "a" stuff then i commit as revision "A". Then i do another "b" stuff and i commit as revision "B", ... When i check what happened with my file in crucible, i can see that there were changes a + b + c by selecting the piont A <-> C.
When i add a new file to the file system with some content, that is commit "A" with modifications "a". Then i do some changes and make some commits. What i would expect from Crucible is that when i select points A < - > C then i get modifications a + b + c. Instead if i select only commit "A" then it shows correctly all "a" modifications. Although when i select A < - > C then it shows only b + c which is - from my point of view - a bad practice. The only notification where from i should realize there is a modification in "a" is that the name of the file is green representing it is a new file.
All the time i was reviewing i thought the part of the code that i should review are the parts highlighted green. As for this example, this is not true at all. Probably we have skipped couple of lines of reviews because this.
I hope explanation is ok so more people can realize what is the problem here. I am also suggesting to all the reviewers to be aware when reviewing a new file. Also waiting for a fix.
Hello everyone!
After analysis we decided that we will use the "Added / Modified" label to show whether the file has been added+modified or only modified between revisions. We do not want to clutter the revision slider with even more data. Therefore, please watch
CRUC-6671for further updates.Kind regards
Marek Parfianowicz
Development Team Lead