Thanks for that detailed reply Denis

Interesting indeed, Matty. I looked at the GA plan of this vessel & I noticed something interesting:
2nd (large hold) has typically 10 hatch covers (from #5 to #12).
On hatch covers 12 & 11 it's possible to place 2 TEU containers or 1 FEU (in length). Same for covers 10 & 11, 8 & 7, 5 & 6. But on cover 6.II it's possible to place only 1 TEU which would extend a bit on cover 6.I, there rest of which would be free of container space.
An even closer look gives that covers 6.I & 6.II are actually shorter than the rest of covers for the 2nd hold. I would guess standard length hatch covers are used for 1st hold (4 covers) & same length covers are used for 2nd hold, but since second hold has such a length which would be impossible to fully close by 9 standard length covers or 10 of them won't fit, they use 2 non-standard length covers 6.I & 6.II.

I notice the hatches marked numerically but why are two marked as 6.1/6.2 ?