Why can't I read a hydrometer correctly??

Things have been going very well for the last half a dozen brews or so. Consistent, decent efficiency, no major mistakes or screw-ups.
Then last night happened.
Get done with the entire sparge process and dipped into the kettle for a hydrometer reading. I fill the tube, set the hydrometer in it, and set it in the freezer for a couple of minutes while I set the kettle on the burner and get the flame going. Exact same thing I always do.
I go get the tube out of the freezer and check the hydrometer and the temperature. Spin it gently to make sure it's not sticking. Hydrometer reads 1.020 at 121 degrees. Plug it into promash, that means my preboil gravity is only 1.030?? Crap, that's 12 points low. I check my reading again. Yup. Hydrometer's not stuck, still reading the same. That puts me at about 50% efficiency?
So I'm pissed off, can't figure out what I did wrong in my mash, and add a pound of light DME to my boil and go on with the brewday.
Postboil gravity, taken from the end of the transfer to the carboy, and holy **** that hydrometer's floating high...1.076?? Now I've overshot my gravity 27 points??

I have no idea how I screwed this up. Obviously my preboil reading was off, but why? Good lord I'm retarded.

Over 100° your hydrometer may not be reading correctly even with corrections for temperature. Try putting your pre-boil sample in a wide mouth shallow container to cool it down before adding it to the tube.

Nurmey said:

Over 100° your hydrometer may not be reading correctly even with corrections for temperature. Try putting your pre-boil sample in a wide mouth shallow container to cool it down before adding it to the tube.

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Or stick it in the fridge for awhile to cool down....

Or get a refractometer for the early gravities, you can get one drop cool enough to read almost instantly.

Oops! Yes, cool it in the fridge in the shallow container.

My guess is that you didn't stir the wort and you were reading the lower gravity sparge that rested on top of the higher gravity first runnings. Stir the kettle well before taking a sample.

Nurmey said:

Oops! Yes, cool it in the fridge in the shallow container.

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make it a small stainless bowl. Metal conducts heat, while plastic and ceramic are insulators.