I have been told by a curator at the Soldiers of Gloucestershire Museum that the tin was designed to hold two helmets. His explanation follows -
The tin should probably have some internal fittings, viz. - a hinged shelf about half way down the body of the tin with a small rest opposite for the shelf to rest on; a lip about two inches wide, six inches long and three inches down from the top of the tin, with a hinged flap opposite. At the bottom of the tin might rest helmet fittings, other kit, etc. On the middle hinged shelf would rest a helmet, and on the lip and flap above, another helmet. Thus the top of the bottom helmet is inside the helmet above it, and the top of the uppermost helmet fits snugly inside the lid. We have several of this description but only one with what appear to be contemporary helmets, in this instance, Wolseley pattern.
I don't remember any such internal fittings and I don't remember noting whether they had been removed but the above explanation seems entirely plausible.
Stuart