Most modern devices don't give you direct access to the sdcard from your computer. The "sdcard" is just an emulated storage area which can not be separately unmounted and provided to your computer via USB. They explained this sometime ago when it was introduced with 4.0? 4.1?
What you see on your computer is what the Android Media-Content-Provider knows.
In your case it's not up to date with the current content. To get it syncronized you will have to trigger a media scan.
See this question for how: How do I refresh/rescan the SD memory in Android 4.4 KitKatHow do I refresh/rescan the SD memory in Android 4.4 KitKat
If a normal media scan does not bring it into a consistent state:
There is a system app called similar to "Media storage" (packagename com.android.providers.media on a Nexus5), you can clear it's data and then do a media scan to start from a clean state.