Critical Strike Chance: How is it computed?

You have 5% base critical strike chance.

If you have 100% increased critical strike chance, you will now have 10% critical strike chance.

If you then added on +5% critical strike chance, you would be up to 20% critical strike chance.

(5 + 5) x (1 + 1) = 20

Percentage increases of a percentage value can be confusing.

I think the spell crit was probably flat and didn't have the word "increased" but I'm not sure. Do you have an example in a screenshot or link on last epoch tools? Edit: The only other explanation that I can think of is that you've got some source of "more spell crit chance". That could cause this to spike harder because it's multiplicative with increases. However, I'm pretty sure that doesn't exist anywhere in the game. (I could be wrong on that though)

I'm also not sure why when you take off all your gear, you still have 13% crit without seeing the character. I suspect that you might have had some passives or maybe a monolith blessing that was giving you some crit possibly. You can use last epoch tools to load in your character from offline save or online lookup on their site and link it and we could probably find it.