Updating Large Apps Prompts Full Download

Hi,

Whenever I get a large app update via the Mac App Store it appears to download the update, shows as installing and then it proceeds to download the app in full instead of applying the update. It's done it many times in the past but just updating Final Cut Pro X now and it download over 400MB for the update, showed as installing and now it's redownloading the whole app at 2.82GB. It did the same for Compressor too, downloaded about 300MB for the update, showed as installing and then proceeded to download and install the whole app again.

Is there any reason why this might be happening or anything I can do to prevent it from happening?