When you have a chest of 13,20,27,34,41,48,55 long you would expect the capacity of 32 * length_chest. But it actually 12,18,24,30,36,42,48 repectively. So for 55 this would be 8 * a_chest_of_6 + 7 * "connector_chests" which do not add to the total capacity.

I don't know if it is as intended and I don't mind it, the chest are still great to use. Just wierd that's all.

This is correct.
I guess my thought process was something along the lines of;
"I don't want to change how good this is compared to single chests next to the train for loading/unloading (in terms of capacity), so I'll keep the capacity the same."
There isn't really a better reason than that, sorry for the confusion.