serious..its like pay from a job, you don't get it when you start, you get it after youve' been there for a week....read the info on the screen...it tells you this quite clearly...
the actual shields is similar... 4 weeks....not 1, not 2...but after 4 weeks.
and its not a decay rate, its a membership of an allied force, you pay for certain privileges and it just happens that there are levels below 3, or 4 apparently in your case that cost smaller amounts so if you do not contribute every week, the credits all get used up in the lower levels till you either pay up again or remain at level 1.
Level 1 is the only level where you don't have to pay every week. and you still get 1,000 credits each week just for being at level 1 without contributing in any way.