"We are well aware that the labeling of the seasons on these DVDs are confusing, but for some reason FOX in the UK has decided to categorizes the episodes differently to FOX in the States."
"Originally "Family Guy" was only available on DVD in the UK and were sold as seasons 1, 2, and 3. Then the States caught up by selling those seasons as two volumes. Since then FOX has opted for global releases, which seems to have resulted in the UK DVDs (as well other non region 1 areas) continuing to be labelled as seasons despite not being complete seasons."
There's really is no reason for confusion when it comes to the dvd sets.
After Season 3, the sets have been released as identical "Volumes" for all regions but remain labeled as "Seasons" (or "Series") outside R1, despite NOT being complete seasons. In fact with the exception of Season 3, there has never been a complete season set for any region.
All information below is based on how Family Guy aired originally & presently on FOX in the U.S.
Region 1 ......................................................Region 2,3,4
"Volume 1" (Complete Seasons 1 & 2) = "Season 1" (Complete Season 1 & Seven episodes of Season 2)
"Volume 2" (Complete Season 3) = "Season 2" (Remaining 14 episodes of Season 2)
N/A = "Season 3" (Complete Season 3)
(Sets now become identical but named differently)
"Volume 3" (First 13 episodes of Season 4) = "Season 4" (First 13 episodes of Season 4)
"Volume 4" (Remaining 14 episodes of Season 4 (minus "Stewie B. Goode", "Bango Was His Name-O" & "Stu & Stewie's Excellent Adventure")) = "Season 5" (Remaining 14 episodes of Season 4 (minus "Stewie B. Goode", "Bango Was His Name-O" & "Stu & Stewie's Excellent Adventure"))
"Volume 5" (First 13 episodes of Season 5) = "Season 6" (First 13 episodes of Season 5)
"Volume 6" (Remaining 5 episodes of Season 5 and the first 7 episodes of Season 6 (minus "Blue Harvest")) = "Season 7" (Remaining 5 episodes of Season 5 and the first 7 episodes of Season 6 (minus "Blue Harvest"))
"Volume 7" ("Back to the Woods", "Play it Again Brian", "The Former Life of Brian" & "Long John Peter" and Season 7's episodes - "Love Blactually" to "The Juice Is Loose!") = "Season 8" ("Back to the Woods", "Play it Again Brian", "The Former Life of Brian" & "Long John Peter" and Season 7's episodes - "Love Blactually" to "The Juice Is Loose!")
Next Set: Release date TBA
"Volume 8" (Season 7's episodes: "FOX-y Lady", "Not All Dogs Go to Heaven", "420", "Stew-Roids", "We Love You Conrad", "Three Kings", "Peter's Progress" & (presumably) Season 8's episodes: "Road to the Multiverse", "Family Goy", "Spies Reminiscent of Us", "Brian's Got a Brand New Bag", "Hannah Banana" & "Quagmire's Baby") = "Season 9" (Season 7's episodes: "FOX-y Lady", "Not All Dogs Go to Heaven", "420", "Stew-Roids", "We Love You Conrad", "Three Kings", "Peter's Progress" & (presumably) Season 8's episodes: "Road to the Multiverse", "Family Goy", "Spies Reminiscent of Us", "Brian's Got a Brand New Bag", "Hannah Banana" & "Quagmire's Baby")