What is the subject lenght supported and one strange example

I just noticed something strange.

We have user who opened thread called as:
" Why some of the tapes were not fully used but they were marked "Used full"?"

So, I did hit reply and subject automatically gets set to:
" Re: Why some of the tapes were not fully used but they were marked "Used fu"

OK, we have shift to right for 3 places so I guess first user managed to get subject at its limit and mine "Re: " removed "ll"?" part. But then I see next user who replies and his subject is:
" Re: Why some of the tapes were not fully used but they were marked "Used full"?"

So, bye bye limit theory. Why is that so? Why my replay gets subject cut?