In your Control Panel I believe there is this setting (screens vary slightly but I believe all users have that option):

[Full][Compact][None] How should the reply box in thread view be displayed? (choose 'None' to disable)

I've not played with it much but if the poster has it set to None as I do RE: is used.  I believe it is the same if the setting is Compact.  If it is set to Full, I believe Re: is used.

Frankly, I had not noticed the difference before; I believe it is just on oversight when the thousands of lines of code were written.  Thanks for bringing it up . . . I don't think there will be an attempt to change it.  In fact, I probably should have not made this post; I may be wrong and changes are the responses will be interesting.