How a percentage works is better understood if you have a known example/value.
Like you said, above, “If I have 400 of X, 2% isn’t much”. No, it never will be.
Crit. (chance): Crit. is the scale of how much bigger a crit event is compared to the base event; if base is 100 and Crit is 20%, a crit event will be 120 (Crit chance is how often a Crit will happen; 30% = 3 times in 10 events will be a crit.).
Haste (chance): How much faster events happen; if a base event takes 100 time units to happen, 19% haste will mean it happens in 81 time units (Haste chance is how often an event will be affected by Haste - see Crit chance, above).
Mastery: This is linked to an ability Event specific to the character in question & relates to how much more effective that ability is; if the base ability is 100, then Mastery of 25% will result in either an Event of 125 or 80, depending on whether it increases or decreases what happens.
Versatility: this is a increase/decrease in a core value; so, if core damage received would be 100, 10% reduction would mean 90 happens - at the same time, if core damage you do would be 100 the 10% would make that 110.
On top of all that, you get factors that change the above in some way that it isn’t always clear what they mean,
Example: +5% crit - how does that increase happen? Does that mean my crit goes up by 5 percentage points on top of what I have OR is it 5% of what a total crit is (not the same).
“Increases Crit. %” adds to the overall crit event, with above as an example, 120 goes to 125.
“Increases % Crit.” this adds to the overall Crit event, but as an overall amount of the total Crit event; 5% of a 120 Crit is actually 6, so the overall Crit is 126.
“Increases Crit. chance (or chance to crit; diff. words, same meaning)” adds to how often a Crit occurs, so 30% becomes 35%.
I get that ^^^ is a lot to take in, but I tried my hardest to make it sound easy to understand & I admit I may have failed.
EDIT:
A “Chance” to happen has ZERO bearing on when it last happened; it does NOT mean that 3 of EVERY 10 will happen, but you have a 3/10 chance OF it happening which may mean that it happens 6 times in a row, then not at all for ages… the overall outcome for every time an Event did happen compared to when it didn’t happen are 3 (times it did) to 7 (times it didn’t).