What is going on here? One a/c in focus and one out of focus

Can anyone please tell me what is going on here?
The Demon (rear) looks to be in focus whilst the Hind (front) looks to be slightly out of focus.
At first it looks like a depth of field problem but at that distance with the following settings I thought it unlikely.
At least one of the 9 autofocus points is on each a/c
ISO 200 1/320 sec @ F13 Lens Canon 100/400 set to 115mm

I had this problem at Shoreham trying to picture the two Mustangs in formation.
Could it be simply that one of the a/c is moving relative to the other due to something like bumpy air?
Has anyone else had a similar problem?
Is this a recognised problem of photographing formations?
John