Here is a visualisation of what happened to you, as it has been explained in previous posts;
Merits
Week 1
Week 2
Week 3
Week 4
Week 5
Week 6
Week 7
Gained week 1
10000
5000
2500
1250
0
0
0
Gained week 2
5000
2500
1250
625
0
0
Gained week 3
5000
2500
1250
625
0
Gained week 4
5000
2500
1250
625
Gained week 5
5000
2500
1250
Gained week 6
5000
2500
Carried over
5000
5000
5000
4375
4375
4375
Total
10000
10000
10000
10000
9375
9375
9375
Once you've hit the end of Week 4 suddenly only 4375 merits got carried over (or a bit more if you didn't reach rank 5 from scratch). That's because the merits you earned during that first week that got you to rank 5 stopped being carried over (cf what markkuk said on the first reply).
Here is now the visualisation of what people have been advising in the thread, which is getting 5334 merits a week (in the example I'm using 5336, but that is just for the sake of having whole numbers );
Merits
Week 1
Week 2
Week 3
Week 4
Week 5
Week 6
Week 7
Gained week 1
10000
5000
2500
1250
0
0
0
Gained week 2
5336
2668
1334
667
0
0
Gained week 3
5336
2668
1334
667
0
Gained week 4
5336
2668
1334
667
Gained week 5
5336
2668
1334
Gained week 6
5336
2668
Carried over
5000
5168
5252
4669
4669
4669
Total
10336
10504
10588
10005
10005
With 5334 you would get 10001 total instead of 10005, but you get the idea.
I'm guessing the reason behind this mechanic is to prevent players who abused a bug to gain lots of merits from staying at rank 5 indefinitely, without having the moderation team manually searching for them. But hey, I'm not the devs so my guess is as good as anyone's