How is water boiled in the microwave different than water boiled on a stovetop?

Here's a weird one. Today I wanted a quick cup of instant coffee. My water boiler is broken, and instead of boiling on the stove, I just threw some water in a Pyrex glass and microwaved for about three minutes.

Stirred in coffee, then half-and-half (Trader Joes, real milk/cream, not organic) — and it immediately curdled! The cream completely refused to combine with the coffee at all. The expiration date is still two days from now. Cream tastes, smells fine.

Tried another cup: same result.

Finally, I boiled water on the stovetop and made my coffee. Added the cream with bated breath, and ... a perfect creamy cup of instant coffee.

I do remember hearing somewhere that microwaved water is different than stovetop water, so:

1. How is water boiled in the microwave different than water boiled on a stovetop?


2. How is that is causing my cream to curdle?