Just to be completely clear on the ranking and inactivity. Your ranking is adjusted by the Elo system, a number which is generally around 1500-1700. It is the special guy behind what your global and mod ranks are, you generally get anywhere from +0.50 to +2.00 Elo points per win, and -0.50 to -2.00 for losses. Everyone has an Elo rating, a computer recognizes all these ratings as numbers and sorts them in descending order (1700 obviously being better than 1500).
Anyway, Toribash recognizes your Elo rating and locates where on the list, what your global / mod rank is. Toribash will try it's best to match your Elo rating to an accurate Global ranking. If you have been playing and frequently losing, your Elo will gradually decrease in number. Your Global ranking will not react to this instantly, it will gradually increase upwards to a higher number eg: 1600 Elo rating being 900,000 and you've lost a few matches so your Elo Rating has dropped to 1595. Your Global ranking might only display 900,030 for now, but that's because it hasn't reached your real ranking at that moment in time. Then say you come back online a week later, it's now at 1,200,000 Global Ranking. Surprise! Your Global Ranking has gotten to it's actual rank, which took some time.
Also, as for Inactivity, other players are constantly registering new accounts, which are added to the system, those players get good, and eventually might rank up above you. Therefore dropping your rank by 1 yet again. So after something like a 6 month break, there's plenty more players and it is also likely to cause you to drop in global rank.
If there's anything i missed on that, be sure to let me know. I hope this helps a few of you.
Sparky~
EDIT: forgot to mention, if you'd like to understand the Elo Rating behind the Rankings, check out this page:
http://en.wikipedia.org/wiki/Elo_rating_system