[AZ-AlphaXi]

Where you're going wrong is both chapters don't move to their #22 together. Provided that ABC's 20th name has gotten a bid to XYZ the last stop is open and Suzie as #21 moves onto ABC's first bid list. If DEF's 20th got a bid to PRQ then their #21 (Betty Boop) moves onto their bid list but has DEF listed as her #3. So the matching is attempted again. Suzie still has DEF as #1 and doesn't match and Jackie still isn't on DEF's bid list and Betty still doesn't match either ABC or DEF. This has become grid lock, nobody is going to match, and there's a procedure for breaking this that would end up giving a bid to ABC to Suzie. Depending on what's happening with Betty's first choice she may or may not get the bid to DEF. If she does get a the bid to her first then Jackie moves onto DEF's bid list and gets a bid. If not, Betty may end up getting it, if her first and second choices fill to quota.

Does that make sense? The thing to remember is that once a name has moved onto a chapter's first bid list, it stays there until it is matched or removed when you match elsewhere. PNM names and 1st choices continue to be read until the first choice fills to quota or in the case you have to break grid lock.