There are two types of compression for images: lossless and lossy. Lossless compression will reduce the file size without making any visible changes at all. It simply gets rid of unnecessary data that's causing the image to be bigger, but the image will look exactly the same. Lossy compression reduces the size of the image much more than lossless, but it can also change the appearance of the image.

Imgur uses both types of compressions. Every image that goes in is compressed by lossless compression because there's no reason not to. It reduces the size of the images with no downside. Lossy compression is only used when the image is above a certain file size (5MB for pro, 1MB for standard).

So, your image is mostly like experiencing lossy compression and therefore losing a the original quality. You can upgrade to a pro account to avoid this with images up to 5MB.