Lumia 820 weird storage bug

My phone says I have more than 3gb of storage available.

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When I try to download some apps, such as this one, I keep getting the error saying I don't have enough storage available. I don't get it, I have 3gb free, the app is 775 mb, yet it won't download. Some other big apps (700+MB) have the same issue. Getting rather annoyed.. Anyone have any ideas?