the 2013 results that fell off the rankings:
4-3 W over Germany (friendly)
2-1 W over Jamaica (WCQ)
2-0 W over Panama (WCQ)
1-0 W over Honduras (WCQ)
I can't find the new rankings; it looks to me like they aren't supposed to come out for a couple more days, but it's possible that some Gold Cup wins from 2013 also might be falling off the rankings.
The new rankings are adding 2 draws and 2 wins ( 2 friendlies and 2 WCQ) so the stuff following off was more valuable than the new stuff being added. The FIFA rankings are pretty dumb, but I hope that answers your question. Next month's rankings will include all of the 2017 Gold Cup so we should be bouncing right back up