500D RAW image help, over exposure

Can someone please tell me what is going on in these pictures. I took both pictures with the same camera setting apart from on thing. The normal picture was +0.67ev, while the overexposed picture is +0.33ev (in camera), yet ended up completely different. I had this problem several times today. Some images just ended up way over exposed

I'm quite new here, my camera is 6 months old and I've taken roughly 6000 pictures.

Exif should be intact, why are the pictures so different using the same settings? Can it be a problem with the lens not stopping down? I recently bought a Sigma 10-20mm F3.5 and haven't used it very much, maybe the camera just thought the lens was F16. DOF button works just fine, just tried it. It was quite cold outside when I took the images, around -12c.

same settings for both images (both unaltered in Lightroom) taken just seconds apart while I adjusted EV.

1/60s
F 16

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According to the EXIF, aperture and shutterspeed were identical in both cases: f/16 at 1/60s.

I'm guessing the aperture didn't close on the second image. (Faulty lens)

I have to test more in cold weather, it works fine indoors. Tried everything 1/4000s etc, it always closes indoor. They should work in cold weather as well right? -12c isn't that bad really...

That's pretty extreme, for sure! Yeah, cold weather shouldn't make things totally malfunction (aside from battery life going down) but you never know. Do some extensive testing and if it keeps happening regularly you might consider sending something in. Of course it could be the camera as well as the lens, so if you have more than the one lens test with both.

I just put the camera in a bag on my balcony for an hour. Sure enough the aperture didn't close and it's only -9.5c outside right now. Was an easy test, I just put it in manual and F22. Sometimes it worked, sometimes it didn't. about 50/50.
I'll test it with the kit lens as well, thanks for the help. To say the least I was very confused when I was outside shooting today and couldn't at all understand why some pictures with the same settings were completely overexposed got quite annoyed to say the least

Yep it's definetly the lens, this time I can see the aperture struggling when I set it to F22 and press the DOF button it doesn't want to close. RMA time, I really hope it isn't a design fault since the 10-20mm F3.5 is relativly new... The 18-55 IS works just fine.