Firefox 'About' version numbers and updates question

Hey...

If you go to 'Help' -> 'About ...' in FF:
In the latest FF 3 Portable Beta 5, the version number is:
rv:1.9b5) Gecko/2008-03-26-20 Firefox/3.0b5 ("-"'s added by me)

In my previous installed FF 3 Portable Beta 5pre trunk it was:
rv:1.9pre) Gecko/2008-03-29-04 Minefield/3.0pre ("-"'s added by me)

And in the latest trunk from ftp://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/ it is:
rv:1.9pre) Gecko/2008-04-03-05 Minefield/3.0pre ("-"'s added by me)

I have realized that the first four digits is the year, the next two the month and the next two the day. (=this shows that my "old" (from 29/3) trunk B5pre one (made portable as described in this thread: https://portableapps.com/node/12311 ) is a never version than the final Beta 5 released on this site... (thats odd I think by the way))
Now to the question and purpose of this thread:
I don't understand what the last two digits in the version number stands for/means... can you tell me?

Also I have another question:
Looking on this page: http://www.squarefree.com/burningedge/ ... The dates of the latest releases is supposed to be (for when they have been released) (link found via this site: http://www.mozilla.org/projects/minefield/ ).
Currently the latest trunk was released the '2008-03-26'.
Then how come that on ftp://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/ it says that there has been a modification/change today 3/4?
Are they just uploading the same trunks over and over again, removing the old uploads, though no changes has been made - just doing it with no logical explanation? maybe they think it looks good, that it says the latest modification is present day?
Sum up of question two: Why does it say on (...)mozilla.org(...) that the latest change has been made present date, when Burning Edge says that no new trunk has been made for a week?

I'm just curious...