The rules also state that if the 8th card is a 'Air raid' or 'Morale decline card' it is placed on the discard pile of the following day. So this means a day of defence can potentially also end with 7 rather than 8 cards. Correct?
I didn't read this rule right the first three times I looked at it and had the same thought as you.
Then I realized it is saying that once the 8th card is already in the pile and an Air Raid/Morale card is discarded as the 9th card that it goes into the next day.
To clarify further, the way I learned my lesson was when I ended up with a troop card as a 9th card discarded! [I killed a troop for #8 and then a mine killed one.] What do I do with that 9th card? Assumed it went to the next day but it didn't really say that.
So the real rule would be that any cards beyond 8 go into the next day.