exposure – AE-Lock and Re-compose

I’m confused on how AE-lock works. I was adivsed and also tried this. Walk into a room point at what i want to capture, lock the exposure with AE-lock. re-compose and take the shot. in theory the shot should have the same exposure as when i locked it. I don’t see this.

for example. i went into a dark room and found some light. i metered the light and my Shutter was at 1/200 to see if it was right i metered the dark spot and it was 1/60. so i Ae-locked the 1/200 from the light locked it re-composed and the shot came out dark. my camera focused again while i pressed the shutter all the way down and took whatever shot it wanted. I was in AV mode with a fixed iso.

What happend?

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This question is more about, Why didn’t AE-lock keep the exposure that i metered from and focused again before pressing the shutter all the way down

Any help is appreciated