Talon SS Shot String – Interesting Results

So I did my first shot string but I also shot for accuracy too. So I set the target a 21 yards.

First 10 shots made a nice dime sized group.
BUT THEN!!!
Next 10 shots made 2 groups one same spot as before, and another group about half an inch low.
THEN
next 10 shots were up and down between those two groups.

Now heres the question for all you experts. How is this possible? Because when I look at the shot string I dont see any real deviations to get flyers. All pellets were weighed, parralax was adjusted out and so forth. Should I blame JSB? Should I clean the barrel? Whats your thoughts?

Top Hat: .08
Power Wheel: 5
Pellet JSB Exact 16gn .22
Extreme Spread 34.2

1: 802
2: 800.7
3: 798
4: 795.1
5: 794.6
6: 794
7: 794.7
8: 792.8
9: 791.7
10: 792
11: 791.5
12: 787.1
13: 787.5
14: 789.8
15: 788.1
16: 784.7
17: 778.5
18: 783.5
19: 785.3
20: 783.7
21: 781
22: 778.9
23: 778.6
24: 777.9
25: 776.9
26: 775.2
27: 776.1
28: 773.8
29: 767.9
30: 773.3
31: 769.7
32: 771
33: 767.8