Invincibility - combat attributes
you're first screen shot is missing weave and maneuvers for one thing. If I have calculated things correctly you should be at 39.8 or so defense with a fully saturated invincibility. For a brief moment invincibility will stack with itself, making it look like you get an extra 10.53 defense or so, hence why you see 50% + from time to time. Many buff powers that are toggles work like this - they have a duration that is longer than their pulse. This is to compensate for lag effects.
I am an ebil markeeter and will steal your moneiz ...correction stole your moneiz. I support keeping the poor down because it is impossible to make moneiz in this game.
2 Things
First off, Invincibility has 2 parts to it's defence. a large defense bonus(In your case 5.87%) that is flagged to not stack and a small one(in this case 1.17%) that can stack up to the max targets(10). This means that with one target in range you get the large defense bonus AND 1 of the small defense bonuses.You can then add up to 9 additional targets for the small defense bonus. This is why you see 10 listings for the small bonuses.
Secondly, toggles reapply their effect every half-second or so
The small defense bonus has a duration of 1 second, and the large bonus lasts 1.12 seconds. So as Lohenien said, invincibility has overlap sometimes, which is why you're seeing extra randomly.
The combat attributes window used to show the flux every second or so, but recently it seems to pick the normal or double value randomly for the total, as if the CA window has lag. This is especially noticeable with inspirations or build up and such, it takes a second or so after using them before the CA window will show the damage/to-hit bonus and such. *shrug*
To min/max both my inv tank and brute i was going for the softcap S/L. On a tank you can hit this very easy with the basic powers (weave, CJ, invincibility, tough hide and the global IO). However on my brute, on paper, i can only hit 38%.

So i went ingame to see how things would go, i got into something odd.
My base defense of my brute is 8.87%, 5.87% from tough hide, 3% from global.
Along with that i use CJ, manauvres and weave, all 3xSO slotted, giving me 21.23% base.
I grabbed a AE mission with lotsa melee mobs and see what my numbers did. I should get around 38% percent, as its 5.87% 1st target and 1.17% for the other 9.
Now randomly, my defense goes all the way up to 50% according to Combat Attributes. It does go up/down, it doesnt change, it stays at 50%. So i tried to replicate this, what exactly causes this.
Below my screenshot:
On the left, no toggles, only invicibility. All is correct, 8.87 and 5.87+(~1.17*9), around 26%.
right side, i put on all toggles and pop 2 purple (2x12,5%), see the total: 75.56%.
Yet if you add all shown numbers, it is 63.8 (difference of 11.7%). Weird enough, thats 10 times 1.17%.
Suddenly i have nett 50.56% defense, but thats impossible. I couldnt get that 50.56% screenshot without lucks, by then i died and it took me a while to figure out how to make a screenshot with UI.
Is this just Combat Attribute borking up, it looks like it adds another 12-12.5% to my actualy value (around 38%), wich i cant seem to see in the Combat Attribute. 1 weird thing i did see, it has 10 lines of Invincibility 1.17% and 1 line 5.87, but shouldnt this be only 9 lines? As the power says 'up to 10 mobs'.
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