Deck gun hitting way off target

[Platapus]

Getting back to SH4 after a break (good to be back)

Playing patch 1.4 with Trigger Maru Mod

Every once in a while, seemingly independent of our relative speed, my deck gun's shells impact nowhere near where I am aiming.

Target was <900 yards with a speed of 6 knots. I was matching 6 knots along side.

I had my gun scope at the first zoom (low power) I had to put the centerline of the ship at the extreme edge of my scope viewfield in order to hit the ship.

Does this sound right?

Naturally, I realize that there will be a need to "lead" the target with the guns but to that extreme?

What is even more strange is that other times my shells hit very near the aim point.

What am I doing wrong?

Nooblet minds want to know