Block Dissolve inaccurate?

I have a grid of images 960x720 in a grid in my 16K comp (15360x6480)

I have block dissolve set to 960x720

But it is not matching the 960 width

there are 16 across which is an even number but it is the width that is inaccurate see attached?

Is it broken? Did it ever work correctly? I am sure I used to use it accurately

( plus the standard moan...STOP FORCING ME TO TAG AND STOP LIMITING THE SUBJECT LENGTH NOW COME ON!!!! )

Tris