Since as an european the metric system is a bit hard to understand. 
You mean Imperial or US

We europeans are the ones that use Metric system.
No trouble understanding that.
Anyway the height difference is insignificant:
5 3/8" = 13,6525 cm
5 1/4" = 13,335 cm
So the difference is 3,175 mm
Width difference is more: 6,35 mm, but that means you can store thicker stuff in there.
But yeah why oh why don't these come with sleeve.