pref and bid day

That is a good explanation above but I'll add a little detail if you're still confused. Ok, so you ranked (examples)
1) ABC
2) DEF
3) XYZ
The computer first attempted to match you with ABC, as they were your first choice. Sadly, they filled quota before they reached your name on their list. So then the computer attempted to match you with DEF, and they also filled quota before reaching your name on their list. So, the computer then attempted to match you with XYZ, and you were high enough up on their list that they hadn't matched quota when they reached your name, so you received a bid to XYZ.
"Guaranteed placement" does NOT mean you're guarenteed a bid to your 1st/2nd choice; it just means that if you list ALL of the groups you preffed, you will receive a bid somewhere (through regular matching or as a QA to reward you for maximizing your options).
Note-not ranking XYZ would've meant you wouldn't have gotten a bid at all. Once you are on a group's first bid list, you stay there until you match somewhere else (because you ranked them higher) and then the first girl on their second list will move up on to their first list. So you were not on the first bid list for ABC or DEF, because they filled quota before reaching your name. Please give XYZ a chance! They saw a sister in you, and the others may have really liked you, but you weren't high enough up on their lists. I know you're disappointed but please stick it out and get to know your new sisters. Good luck!