This can definitely happen when some songs from and album match and others upload. If all songs on a given album matched, this is strange. There is an option in iTunes, under the Edit menu and Preferences to normalize the volume of all songs. Once the preferences dialog is displayed, it on the Playback tab. Check the box to enable 'Sound Check.' iTunes will start analyzing all your songs. This may take some time when first turned on. iTunes is not modifying your actual song files, but just the meta-information it stores about them. It will store a dB level offset for each song. When it plays back songs, it will use this offset to approximately make all songs the same volume.
Note, after enabling this option in iTunes, you will also need to enable this option on your other devices as well, like you iPhone. The option is under Settings and Music.