Your gold income is regularly 100%. When you get the +25% increase, as it is your only bonus, your new gold income is 125%. When you get that +100%, it multiplies instead of adds, like this:
+25% = (1 + 0.25) = 1.25.
+100% = (1 + 1) = 2.0.
1.25 * 2 = 2.5, or a 250% increase in gold.
If you had an additional +75% bonus besides your +25%, it would be like this:
(1.25 + 1.75) * 2.0 = 400%
As it seems in D3 (I do not actually play it) that your bonuses normally are applied additively. If all three of these numbers were to be applied multiplicatively, for example, it would be as follows:
1.25 * 2.0 * 1.75 = 4.375 = 437.5%.
This is a lot more than if you were to add the numbers together for just a 200% increase.
Multiplicative stacking allows for your gold increase to increase much more quickly than with additive.
This is most often used in games for reducing effects though, not increasing. A big example of this would be League of Legends, where things like Crowd Control reductions stack multiplicative as well.
In that case, you have two items that reduce a number by 10%. The formula is like:
-10% = (1 - 0.1) = .9 = 90%.
0.9 * 0.9 = 0.81. (1 - 0.81) = 0.19 = 19% reduction in time, instead of 10+10 = 20%.
As a final note: It would of been much simpler if Blizzard had just said that your total gold find during this period was doubled.